XML 44 R33.htm IDEA: XBRL DOCUMENT
v3.7.0.1

Segment Information (Details) | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2017 USD ($) | Jun. 30, 2016 USD ($) | Jun. 30, 2017 USD ($) segment | Jun. 30, 2016 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($)
Segment Reporting Information [Line Items] |  |  |  |  |  |
Number of operating segments | segment |  |  | 3 |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Total operating revenues | $ 526,501,000 | $ 456,212,000 | $ 2,294,855,000 | $ 2,021,005,000 |  |
Purchased gas cost | 114,176,000 | 64,583,000 | 852,975,000 | 682,265,000 |  |
Operation and maintenance expense | 128,690,000 | 131,388,000 | 385,867,000 | 379,073,000 |  |
Depreciation and amortization expense | 80,023,000 | 72,880,000 | 234,648,000 | 214,927,000 |  |
Taxes, other than income, expense | 62,948,000 | 58,965,000 | 185,611,000 | 171,959,000 |  |
Operating income | 140,664,000 | 128,396,000 | 635,754,000 | 572,781,000 |  |
Miscellaneous income (expense) | (289,000) | 1,118,000 | (450,000) | (90,000) |  |
Interest charges | 28,498,000 | 27,679,000 | 86,472,000 | 84,775,000 |  |
Income (loss) from continuing operations before income taxes | 111,877,000 | 101,835,000 | 548,832,000 | 487,916,000 |  |
Income tax expense (benefit) | 41,069,000 | 35,692,000 | 201,974,000 | 177,224,000 |  |
Income from continuing operations | 70,808,000 | 66,143,000 | 346,858,000 | 310,692,000 |  |
Income from discontinued operations, net of tax | 0 | 5,050,000 | 13,710,000 | 5,172,000 |  |
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax |  |  | 2,716,000 | 0 |  |
Net income (loss) | 70,808,000 | 71,193,000 | 360,568,000 | 315,864,000 |  |
Capital expenditures | 252,763,000 | 253,684,000 | 812,148,000 | 789,688,000 |  |
Income from discontinued operations, net of tax, before gain on disposal | 0 | 5,050,000 | 10,994,000 | 5,172,000 |  |
ASSETS |  |  |  |  |  |
Net property, plant and equipment | 8,924,381,000 |  | 8,924,381,000 |  | $ 8,268,606,000 |
Investment in subsidiaries | 0 |  | 0 |  | 0 |
Current assets |  |  |  |  |  |
Cash and cash equivalents | 69,777,000 | 66,206,000 | 69,777,000 | 66,206,000 | 47,534,000 | $ 28,653,000
Current assets held for sale | 0 |  | 0 |  | 151,117,000 |
Other current assets | 464,605,000 |  | 464,605,000 |  | 483,035,000 |
Intercompany receivables | 0 |  | 0 |  | 0 |
Total current assets | 534,382,000 |  | 534,382,000 |  | 681,686,000 |
Goodwill | 729,673,000 |  | 729,673,000 |  | 726,962,000 |
Noncurrent assets held for sale | 0 |  | 0 |  | 28,616,000 |
Deferred charges and other assets | 310,339,000 |  | 310,339,000 |  | 305,019,000 |
Total assets | 10,498,775,000 |  | 10,498,775,000 |  | 10,010,889,000 |
CAPITALIZATION AND LIABILITIES |  |  |  |  |  |
Shareholders' equity | 3,901,710,000 |  | 3,901,710,000 |  | 3,463,059,000 |
Long-term debt | 3,066,734,000 |  | 3,066,734,000 |  | 2,188,779,000 |
Total capitalization | 6,968,444,000 |  | 6,968,444,000 |  | 5,651,838,000 |
Current liabilities |  |  |  |  |  |
Current maturities of long-term debt | 0 |  | 0 |  | 250,000,000 |
Short-term debt | 258,573,000 |  | 258,573,000 |  | 829,811,000 |
Current liabilities held for sale | 0 |  | 0 |  | 72,900,000 |
Other current liabilities | 487,086,000 |  | 487,086,000 |  | 635,570,000 |
Intercompany payables | 0 |  | 0 |  | 0 |
Total current liabilities | 745,659,000 |  | 745,659,000 |  | 1,788,281,000 |
Deferred income taxes | 1,853,564,000 |  | 1,853,564,000 |  | 1,603,056,000 |
Regulatory cost of removal obligation | 457,060,000 |  | 457,060,000 |  | 424,281,000 |
Pension and postretirement liabilities | 304,919,000 |  | 304,919,000 |  | 297,743,000 |
Noncurrent liabilities held for sale | 0 |  | 0 |  | 316,000 |
Deferred credits and other liabilities | 169,129,000 |  | 169,129,000 |  | 245,374,000 |
Total shareholders' equity and liabilities | 10,498,775,000 |  | 10,498,775,000 |  | 10,010,889,000 |
Reportable Subsegments [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Revenues | 526,501,000 | 456,212,000 | 2,294,855,000 | 2,021,005,000 |  |
Intersubsegment Eliminations [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Revenues | 0 | 0 | 0 | 0 |  |
Distribution Segment [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Total operating revenues | 494,060,000 | 424,905,000 | 2,211,257,000 | 1,936,475,000 |  |
Purchased gas cost | 197,767,000 | 147,569,000 | 1,106,209,000 | 912,231,000 |  |
Operation and maintenance expense | 99,631,000 | 101,819,000 | 296,048,000 | 294,154,000 |  |
Depreciation and amortization expense | 62,760,000 | 59,193,000 | 185,219,000 | 174,748,000 |  |
Taxes, other than income, expense | 56,850,000 | 52,662,000 | 165,032,000 | 153,198,000 |  |
Operating income | 77,052,000 | 63,662,000 | 458,749,000 | 402,144,000 |  |
Miscellaneous income (expense) | (62,000) | 1,243,000 | 334,000 | 804,000 |  |
Interest charges | 18,394,000 | 18,677,000 | 56,437,000 | 57,481,000 |  |
Income (loss) from continuing operations before income taxes | 58,596,000 | 46,228,000 | 402,646,000 | 345,467,000 |  |
Income tax expense (benefit) | 22,082,000 | 15,867,000 | 149,623,000 | 126,090,000 |  |
Income from continuing operations |  | 30,361,000 | 253,023,000 | 219,377,000 |  |
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax |  |  | 0 |  |  |
Net income (loss) | 36,514,000 | 30,361,000 | 253,023,000 | 219,377,000 |  |
Capital expenditures | 205,780,000 | 187,470,000 | 636,449,000 | 528,063,000 |  |
Income from discontinued operations, net of tax, before gain on disposal |  | 0 | 0 | 0 |  |
ASSETS |  |  |  |  |  |
Net property, plant and equipment | 6,678,875,000 |  | 6,678,875,000 |  | 6,208,465,000 |
Investment in subsidiaries | 798,994,000 |  | 798,994,000 |  | 768,415,000 |
Current assets |  |  |  |  |  |
Cash and cash equivalents | 69,777,000 |  | 69,777,000 |  | 22,117,000 |
Current assets held for sale |  |  |  |  | 0 |
Other current assets | 437,700,000 |  | 437,700,000 |  | 489,963,000 |
Intercompany receivables | 983,866,000 |  | 983,866,000 |  | 971,665,000 |
Total current assets | 1,491,343,000 |  | 1,491,343,000 |  | 1,483,745,000 |
Goodwill | 586,661,000 |  | 586,661,000 |  | 583,950,000 |
Noncurrent assets held for sale |  |  |  |  | 0 |
Deferred charges and other assets | 280,240,000 |  | 280,240,000 |  | 277,240,000 |
Total assets | 9,836,113,000 |  | 9,836,113,000 |  | 9,321,815,000 |
CAPITALIZATION AND LIABILITIES |  |  |  |  |  |
Shareholders' equity | 3,901,710,000 |  | 3,901,710,000 |  | 3,463,059,000 |
Long-term debt | 3,066,734,000 |  | 3,066,734,000 |  | 2,188,779,000 |
Total capitalization | 6,968,444,000 |  | 6,968,444,000 |  | 5,651,838,000 |
Current liabilities |  |  |  |  |  |
Current maturities of long-term debt |  |  |  |  | 250,000,000 |
Short-term debt | 258,573,000 |  | 258,573,000 |  | 829,811,000 |
Current liabilities held for sale |  |  |  |  | 0 |
Other current liabilities | 451,026,000 |  | 451,026,000 |  | 605,790,000 |
Intercompany payables | 0 |  | 0 |  | 0 |
Total current liabilities | 709,599,000 |  | 709,599,000 |  | 1,685,601,000 |
Deferred income taxes | 1,251,528,000 |  | 1,251,528,000 |  | 1,055,348,000 |
Regulatory cost of removal obligation | 432,531,000 |  | 432,531,000 |  | 397,162,000 |
Pension and postretirement liabilities | 304,919,000 |  | 304,919,000 |  | 297,743,000 |
Noncurrent liabilities held for sale |  |  |  |  | 0 |
Deferred credits and other liabilities | 169,092,000 |  | 169,092,000 |  | 234,123,000 |
Total shareholders' equity and liabilities | 9,836,113,000 |  | 9,836,113,000 |  | 9,321,815,000 |
Distribution Segment [Member] | Reportable Subsegments [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Revenues | 493,738,000 | 424,553,000 | 2,210,221,000 | 1,935,421,000 |  |
Distribution Segment [Member] | Intersubsegment Eliminations [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Revenues | 322,000 | 352,000 | 1,036,000 | 1,054,000 |  |
Pipeline and Storage Segment [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Total operating revenues | 117,283,000 | 113,855,000 | 339,207,000 | 314,424,000 |  |
Purchased gas cost | 1,251,000 | (438,000) | 2,331,000 | (72,000) |  |
Operation and maintenance expense | 29,059,000 | 29,569,000 | 89,863,000 | 84,919,000 |  |
Depreciation and amortization expense | 17,263,000 | 13,687,000 | 49,429,000 | 40,179,000 |  |
Taxes, other than income, expense | 6,098,000 | 6,303,000 | 20,579,000 | 18,761,000 |  |
Operating income | 63,612,000 | 64,734,000 | 177,005,000 | 170,637,000 |  |
Miscellaneous income (expense) | (227,000) | (125,000) | (784,000) | (894,000) |  |
Interest charges | 10,104,000 | 9,002,000 | 30,035,000 | 27,294,000 |  |
Income (loss) from continuing operations before income taxes | 53,281,000 | 55,607,000 | 146,186,000 | 142,449,000 |  |
Income tax expense (benefit) | 18,987,000 | 19,825,000 | 52,351,000 | 51,134,000 |  |
Income from continuing operations |  | 35,782,000 | 93,835,000 | 91,315,000 |  |
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax |  |  | 0 |  |  |
Net income (loss) | 34,294,000 | 35,782,000 | 93,835,000 | 91,315,000 |  |
Capital expenditures | 46,983,000 | 66,108,000 | 175,699,000 | 261,446,000 |  |
Income from discontinued operations, net of tax, before gain on disposal |  | 0 | 0 | 0 |  |
ASSETS |  |  |  |  |  |
Net property, plant and equipment | 2,245,506,000 |  | 2,245,506,000 |  | 2,060,141,000 |
Investment in subsidiaries | 13,851,000 |  | 13,851,000 |  | 13,854,000 |
Current assets |  |  |  |  |  |
Cash and cash equivalents | 0 |  | 0 |  | 0 |
Current assets held for sale |  |  |  |  | 0 |
Other current assets | 29,265,000 |  | 29,265,000 |  | 39,078,000 |
Intercompany receivables | 0 |  | 0 |  | 0 |
Total current assets | 29,265,000 |  | 29,265,000 |  | 39,078,000 |
Goodwill | 143,012,000 |  | 143,012,000 |  | 143,012,000 |
Noncurrent assets held for sale |  |  |  |  | 0 |
Deferred charges and other assets | 30,099,000 |  | 30,099,000 |  | 27,779,000 |
Total assets | 2,461,733,000 |  | 2,461,733,000 |  | 2,283,864,000 |
CAPITALIZATION AND LIABILITIES |  |  |  |  |  |
Shareholders' equity | 812,845,000 |  | 812,845,000 |  | 715,672,000 |
Long-term debt | 0 |  | 0 |  | 0 |
Total capitalization | 812,845,000 |  | 812,845,000 |  | 715,672,000 |
Current liabilities |  |  |  |  |  |
Current maturities of long-term debt |  |  |  |  | 0 |
Short-term debt | 0 |  | 0 |  | 0 |
Current liabilities held for sale |  |  |  |  | 0 |
Other current liabilities | 38,420,000 |  | 38,420,000 |  | 39,911,000 |
Intercompany payables | 983,866,000 |  | 983,866,000 |  | 957,526,000 |
Total current liabilities | 1,022,286,000 |  | 1,022,286,000 |  | 997,437,000 |
Deferred income taxes | 602,036,000 |  | 602,036,000 |  | 543,390,000 |
Regulatory cost of removal obligation | 24,529,000 |  | 24,529,000 |  | 27,119,000 |
Pension and postretirement liabilities | 0 |  | 0 |  | 0 |
Noncurrent liabilities held for sale |  |  |  |  | 0 |
Deferred credits and other liabilities | 37,000 |  | 37,000 |  | 246,000 |
Total shareholders' equity and liabilities | 2,461,733,000 |  | 2,461,733,000 |  | 2,283,864,000 |
Pipeline and Storage Segment [Member] | Reportable Subsegments [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Revenues | 32,763,000 | 31,659,000 | 84,634,000 | 85,584,000 |  |
Pipeline and Storage Segment [Member] | Intersubsegment Eliminations [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Revenues | 84,520,000 | 82,196,000 | 254,573,000 | 228,840,000 |  |
Natural Gas Marketing Segment [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Total operating revenues | 0 | 0 | 0 | 0 |  |
Purchased gas cost | 0 | 0 | 0 | 0 |  |
Operation and maintenance expense | 0 | 0 | 0 | 0 |  |
Depreciation and amortization expense | 0 | 0 | 0 | 0 |  |
Taxes, other than income, expense | 0 | 0 | 0 | 0 |  |
Operating income | 0 | 0 | 0 | 0 |  |
Miscellaneous income (expense) | 0 | 0 | 0 | 0 |  |
Interest charges | 0 | 0 | 0 | 0 |  |
Income (loss) from continuing operations before income taxes | 0 | 0 | 0 | 0 |  |
Income tax expense (benefit) | 0 | 0 | 0 | 0 |  |
Income from continuing operations |  | 0 | 0 | 0 |  |
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax |  |  | 2,716,000 |  |  |
Net income (loss) | 0 | 5,050,000 | 13,710,000 | 5,172,000 |  |
Capital expenditures | 0 | 106,000 | 0 | 179,000 |  |
Income from discontinued operations, net of tax, before gain on disposal |  | 5,050,000 | 10,994,000 | 5,172,000 |  |
ASSETS |  |  |  |  |  |
Net property, plant and equipment | 0 |  | 0 |  | 0 |
Investment in subsidiaries | 0 |  | 0 |  | 0 |
Current assets |  |  |  |  |  |
Cash and cash equivalents | 0 |  | 0 |  | 25,417,000 |
Current assets held for sale |  |  |  |  | 162,508,000 |
Other current assets | 0 |  | 0 |  | 5,000 |
Intercompany receivables | 0 |  | 0 |  | 0 |
Total current assets | 0 |  | 0 |  | 187,930,000 |
Goodwill | 0 |  | 0 |  | 0 |
Noncurrent assets held for sale |  |  |  |  | 28,785,000 |
Deferred charges and other assets | 0 |  | 0 |  | 0 |
Total assets | 0 |  | 0 |  | 216,715,000 |
CAPITALIZATION AND LIABILITIES |  |  |  |  |  |
Shareholders' equity | 0 |  | 0 |  | 66,597,000 |
Long-term debt | 0 |  | 0 |  | 0 |
Total capitalization | 0 |  | 0 |  | 66,597,000 |
Current liabilities |  |  |  |  |  |
Current maturities of long-term debt |  |  |  |  | 0 |
Short-term debt | 0 |  | 0 |  | 35,000,000 |
Current liabilities held for sale |  |  |  |  | 81,908,000 |
Other current liabilities | 0 |  | 0 |  | 3,263,000 |
Intercompany payables | 0 |  | 0 |  | 14,139,000 |
Total current liabilities | 0 |  | 0 |  | 134,310,000 |
Deferred income taxes | 0 |  | 0 |  | 4,318,000 |
Regulatory cost of removal obligation | 0 |  | 0 |  | 0 |
Pension and postretirement liabilities | 0 |  | 0 |  | 0 |
Noncurrent liabilities held for sale |  |  |  |  | 316,000 |
Deferred credits and other liabilities | 0 |  | 0 |  | 11,174,000 |
Total shareholders' equity and liabilities | 0 |  | 0 |  | 216,715,000 |
Natural Gas Marketing Segment [Member] | Reportable Subsegments [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Revenues | 0 | 0 | 0 | 0 |  |
Natural Gas Marketing Segment [Member] | Intersubsegment Eliminations [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Revenues | 0 | 0 | 0 | 0 |  |
Intersegment Elimination [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Total operating revenues | (84,842,000) | (82,548,000) | (255,609,000) | (229,894,000) |  |
Purchased gas cost | (84,842,000) | (82,548,000) | (255,565,000) | (229,894,000) |  |
Operation and maintenance expense | 0 | 0 | (44,000) | 0 |  |
Depreciation and amortization expense | 0 | 0 | 0 | 0 |  |
Taxes, other than income, expense | 0 | 0 | 0 | 0 |  |
Operating income | 0 | 0 | 0 | 0 |  |
Miscellaneous income (expense) | 0 | 0 | 0 | 0 |  |
Interest charges | 0 | 0 | 0 | 0 |  |
Income (loss) from continuing operations before income taxes | 0 | 0 | 0 | 0 |  |
Income tax expense (benefit) | 0 | 0 | 0 | 0 |  |
Income from continuing operations |  | 0 | 0 | 0 |  |
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax |  |  | 0 |  |  |
Net income (loss) | 0 | 0 | 0 | 0 |  |
Capital expenditures | 0 | 0 | 0 | 0 |  |
Income from discontinued operations, net of tax, before gain on disposal |  | 0 | 0 | 0 |  |
ASSETS |  |  |  |  |  |
Net property, plant and equipment | 0 |  | 0 |  | 0 |
Investment in subsidiaries | (812,845,000) |  | (812,845,000) |  | (782,269,000) |
Current assets |  |  |  |  |  |
Cash and cash equivalents | 0 |  | 0 |  | 0 |
Current assets held for sale |  |  |  |  | (11,391,000) |
Other current assets | (2,360,000) |  | (2,360,000) |  | (46,011,000) |
Intercompany receivables | (983,866,000) |  | (983,866,000) |  | (971,665,000) |
Total current assets | (986,226,000) |  | (986,226,000) |  | (1,029,067,000) |
Goodwill | 0 |  | 0 |  | 0 |
Noncurrent assets held for sale |  |  |  |  | (169,000) |
Deferred charges and other assets | 0 |  | 0 |  | 0 |
Total assets | (1,799,071,000) |  | (1,799,071,000) |  | (1,811,505,000) |
CAPITALIZATION AND LIABILITIES |  |  |  |  |  |
Shareholders' equity | (812,845,000) |  | (812,845,000) |  | (782,269,000) |
Long-term debt | 0 |  | 0 |  | 0 |
Total capitalization | (812,845,000) |  | (812,845,000) |  | (782,269,000) |
Current liabilities |  |  |  |  |  |
Current maturities of long-term debt |  |  |  |  | 0 |
Short-term debt | 0 |  | 0 |  | (35,000,000) |
Current liabilities held for sale |  |  |  |  | (9,008,000) |
Other current liabilities | (2,360,000) |  | (2,360,000) |  | (13,394,000) |
Intercompany payables | (983,866,000) |  | (983,866,000) |  | (971,665,000) |
Total current liabilities | (986,226,000) |  | (986,226,000) |  | (1,029,067,000) |
Deferred income taxes | 0 |  | 0 |  | 0 |
Regulatory cost of removal obligation | 0 |  | 0 |  | 0 |
Pension and postretirement liabilities | 0 |  | 0 |  | 0 |
Noncurrent liabilities held for sale |  |  |  |  | 0 |
Deferred credits and other liabilities | 0 |  | 0 |  | (169,000) |
Total shareholders' equity and liabilities | (1,799,071,000) |  | (1,799,071,000) |  | $ (1,811,505,000) |
Intersegment Elimination [Member] | Reportable Subsegments [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Revenues | 0 | 0 | 0 | 0 |  |
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member] |  |  |  |  |  |
Segment Reporting Information Profit Loss [Abstract] |  |  |  |  |  |
Revenues | $ (84,842,000) | $ (82,548,000) | $ (255,609,000) | $ (229,894,000) |  |

X
- Definition Income from discontinued operations, net of tax, before gain on disposal + References No definition available. + Details
Name: | ato_Incomefromdiscontinuedoperationsnetoftaxbeforegainondisposal
Namespace Prefix: | ato_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amounts due to other operating segments of the same entity attributed to the reportable segment. + References No definition available. + Details
Name: | ato_IntercompanyPayables
Namespace Prefix: | ato_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amounts due from other operating segments of the same entity attributed to the reportable segment. + References No definition available. + Details
Name: | ato_IntercompanyReceivables
Namespace Prefix: | ato_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The cost of investments in securities issued or assumed by subsidiary companies and investment advances to such companies attributed to the reportable segment, including interest accrued thereon when such interest is not subject to current settlement plus the equity in undistributed earnings or losses of such subsidiary companies since acquisition. + References No definition available. + Details
Name: | ato_InvestmentInSubsidaries
Namespace Prefix: | ato_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Reflects the total obligation as of the balance sheet date to remove regulatory assets from service. + References No definition available. + Details
Name: | ato_RegulatoryCostOfRemovalObligation
Namespace Prefix: | ato_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Aggregate carrying amount, as of the balance sheet date, of current assets not separately disclosed in the segment balance sheet. Current assets are expected to be realized or consumed within one year (or the normal operating cycle, if longer). + References No definition available. + Details
Name: | ato_SegmentReportingInformationOtherCurrentAssets
Namespace Prefix: | ato_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Aggregate carrying amount, as of the balance sheet date, of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered and of liabilities not separately disclosed in the segment balance sheet. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer). + References No definition available. + Details
Name: | ato_SegmentReportingInformationOtherCurrentLiabilities
Namespace Prefix: | ato_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Aggregate carrying amount, as of the balance sheet date, of noncurrent assets not separately disclosed in the segment balance sheet. Noncurrent assets are expected to be realized or consumed after one year (or the normal operating cycle, if longer). + References No definition available. + Details
Name: | ato_SegmentReportingInformationOtherNoncurrentAssets
Namespace Prefix: | ato_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Aggregate carrying amount, as of the balance sheet date, of noncurrent obligations not separately disclosed in the segment balance sheet. Noncurrent liabilities are expected to be paid after one year (or the normal operating cycle, if longer). + References No definition available. + Details
Name: | ato_SegmentReportingInformationOtherNoncurrentLiabilities
Namespace Prefix: | ato_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(12)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(11)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_Assets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_AssetsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Sum of the carrying amounts as of the balance sheet date of all assets that are expected to be realized in cash, sold, or consumed within one year (or the normal operating cycle, if longer). Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.9) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6801-107765 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765 + Details
Name: | us-gaap_AssetsCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_AssetsCurrentAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount classified as assets attributable to disposal group held for sale or disposed of, expected to be disposed of within one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e1107-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721533-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2941-110230 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5B -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2473-110228 + Details
Name: | us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This element represents the total consolidated (as applicable) capitalization of the entity which is comprised of its long-term debt and equity instruments. The table may be detailed by subsidiary (legal entity) and include information by type of debt or equity detailed by instrument. + References No definition available. + Details
Name: | us-gaap_CapitalizationLongtermDebtAndEquity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3044-108585 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_CashAndCashEquivalentsAtCarryingValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cost of natural gas purchased and associated transportation and storage costs during the period. + References No definition available. + Details
Name: | us-gaap_CostOfNaturalGasPurchases
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount, after deferred tax asset, of deferred tax liability attributable to taxable differences, with jurisdictional netting and classified as noncurrent. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=75116654&loc=d3e31931-109318 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=75116654&loc=d3e31958-109318 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=75116654&loc=d3e31917-109318 + Details
Name: | us-gaap_DeferredTaxLiabilitiesNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 + Details
Name: | us-gaap_DepreciationAndAmortization
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount after tax of gain (loss) not previously recognized resulting from the disposal of a discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=64827811&loc=d3e1361-107760 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3B -URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721525-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5C -Subparagraph (b)(2) -URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721675-107760 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e957-107759 + Details
Name: | us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount classified as assets attributable to disposal group held for sale or disposed of, expected to be disposed of after one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5B -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e1107-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2510-110228 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721533-107759 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2941-110230 + Details
Name: | us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=64834752&loc=SL49117168-202975 + Details
Name: | us-gaap_Goodwill
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount after tax of income (loss) from continuing operations attributable to the parent. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.13) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=68079674&loc=SL4613673-111683 + Details
Name: | us-gaap_IncomeLossFromContinuingOperations
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(1)(i)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
Name: | us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount after tax of income (loss) from a discontinued operation attributable to the parent. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5C -Subparagraph (b)(2) -URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721675-107760 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3B -URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721525-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3A -URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721523-107759 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e957-107759 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e1012-107759 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=64827811&loc=d3e1361-107760 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=68079674&loc=SL4569616-111683 + Details
Name: | us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32639-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Income Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6515339 + Details
Name: | us-gaap_IncomeTaxExpenseBenefit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of the cost of borrowed funds accounted for as interest expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.9) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
Name: | us-gaap_InterestExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of liabilities and equity items, including the portion of equity attributable to noncontrolling interests, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(23)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(25)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(32)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LiabilitiesAndStockholdersEquity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_LiabilitiesAndStockholdersEquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Total obligations incurred as part of normal operations that are expected to be paid during the following twelve months or within one business cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.21) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LiabilitiesCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_LiabilitiesCurrentAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount classified as liabilities attributable to disposal group held for sale or disposed of, expected to be disposed of within one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5B -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e1107-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2473-110228 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721533-107759 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2941-110230 + Details
Name: | us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount classified as liabilities attributable to disposal group held for sale or disposed of, expected to be disposed of beyond one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2510-110228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5B -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2941-110230 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e1107-107759 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721533-107759 + Details
Name: | us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebtCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebtNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The portion of profit or loss for the period, net of income taxes, which is attributable to the parent. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Net Income -URI http://asc.fasb.org/extlink&oid=51831255 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(22)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(18)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Other Comprehensive Income -URI http://asc.fasb.org/extlink&oid=51831270 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(20)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
Name: | us-gaap_NetIncomeLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The net result for the period of deducting operating expenses from operating revenues. + References No definition available. + Details
Name: | us-gaap_OperatingIncomeLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of income (expense) related to nonoperating activities, classified as other. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
Name: | us-gaap_OtherNonoperatingIncomeExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of selling, general and administrative expense classified as other. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.4) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
Name: | us-gaap_OtherSellingGeneralAndAdministrativeExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The cash outflow for purchases of and capital improvements on property, plant and equipment (capital expenditures), software, and other intangible assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3213-108585 + Details
Name: | us-gaap_PaymentsToAcquireProductiveAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This represents the noncurrent liability for underfunded plans recognized in the balance sheet that is associated with the defined benefit pension plans and other postretirement defined benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2417-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2410-114920 + Details
Name: | us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Period end amount of total net PPE. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The total amount of operating revenues recognized during the period. + References No definition available. + Details
Name: | us-gaap_RegulatedAndUnregulatedOperatingRevenue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
Name: | us-gaap_Revenues
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_SegmentReportingInformationProfitLossAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(19)(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(13)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)(a)(1)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_ShortTermBorrowings
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(30)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 4.E) -URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74512-122707 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(31)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_StockholdersEquity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of tax expense classified as other. + References No definition available. + Details
Name: | us-gaap_TaxesOther
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_SubsegmentsConsolidationItemsAxis=us-gaap_ReportableSubsegmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsegmentsConsolidationItemsAxis=us-gaap_IntersubsegmentEliminationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ato_DistributionSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ato_PipelineandStorageSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ato_NaturalGasMarketingSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=us-gaap_IntersegmentEliminationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |