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INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Sep. 30, 2017 USD ($) operating_segment | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($)
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of operating segments | operating_segment |  |  |  |  |  |  |  |  | 2 |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues | $ 258.7 | $ 323.5 | $ 663.4 | $ 495.1 | $ 279.3 | $ 249.3 | $ 609.3 | $ 399.4 | $ 1,740.7 | $ 1,537.3 | $ 1,976.4
Gas Utility |  |  |  |  |  |  |  |  |  |  |
Natural and propane gas |  |  |  |  |  |  |  |  | 570.5 | 492.2 | 882.4
Other operation and maintenance expenses |  |  |  |  |  |  |  |  | 405.0 | 377.5 | 390.6
Depreciation and amortization |  |  |  |  |  |  |  |  | 153.5 | 136.9 | 129.9
Taxes, other than income taxes |  |  |  |  |  |  |  |  | 137.8 | 125.2 | 142.1
Total Gas Utility Operating Expenses |  |  |  |  |  |  |  |  | 1,266.8 | 1,131.8 | 1,545.0
Gas Marketing and other |  |  |  |  |  |  |  |  | 152.2 | 123.2 | 158.9
Total Operating Expenses |  |  |  |  |  |  |  |  | 1,419.0 | 1,255.0 | 1,703.9
Operating Income | 1.9 | $ 50.3 | $ 180.4 | $ 89.1 | (7.7) | $ 35.3 | $ 167.7 | $ 87.0 | 321.7 | 282.3 | 272.5
Net Economic Earnings |  |  |  |  |  |  |  |  | 167.6 | 149.1 | 138.3
Capital Expenditures |  |  |  |  |  |  |  |  | 438.1 | 293.3 | 289.8
ASSETS |  |  |  |  |  |  |  |  |  |  |
Total Assets | 6,546.7 |  |  |  | 6,064.4 |  |  |  | 6,546.7 | 6,064.4 | 5,277.6
Eliminations |  |  |  |  |  |  |  |  |  |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues |  |  |  |  |  |  |  |  | (14.2) | (5.4) | (76.5)
Gas Utility |  |  |  |  |  |  |  |  |  |  |
Natural and propane gas |  |  |  |  |  |  |  |  | (75.4) | (47.5) | (75.2)
Other operation and maintenance expenses |  |  |  |  |  |  |  |  | (4.1) | (1.8) | (1.0)
Depreciation and amortization |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Taxes, other than income taxes |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Total Gas Utility Operating Expenses |  |  |  |  |  |  |  |  | (79.5) | (49.3) | (76.2)
Gas Marketing and other |  |  |  |  |  |  |  |  | 65.3 | 43.9 | (0.3)
Total Operating Expenses |  |  |  |  |  |  |  |  | (14.2) | (5.4) | (76.5)
Operating Income |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Net Economic Earnings |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Capital Expenditures |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
ASSETS |  |  |  |  |  |  |  |  |  |  |
Total Assets | (1,490.2) |  |  |  | (1,161.9) |  |  |  | (1,490.2) | (1,161.9) | (1,116.8)
Other |  |  |  |  |  |  |  |  |  |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues |  |  |  |  |  |  |  |  | 1.4 | 1.6 | 1.7
Gas Utility |  |  |  |  |  |  |  |  |  |  |
Depreciation and amortization expense |  |  |  |  |  |  |  |  | 0.5 | 0.5 | 0.6
ASSETS |  |  |  |  |  |  |  |  |  |  |
Total Assets | 2,239.5 |  |  |  | 1,836.6 |  |  |  | 2,239.5 | 1,836.6 | 1,554.5
Corporate Eliminations |  |  |  |  |  |  |  |  |  |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues |  |  |  |  |  |  |  |  | 6.3 | 3.2 | 2.0
Corporate And Eliminations |  |  |  |  |  |  |  |  |  |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues |  |  |  |  |  |  |  |  | 7.7 | 4.8 | 3.7
Gas Utility |  |  |  |  |  |  |  |  |  |  |
Natural and propane gas |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Other operation and maintenance expenses |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Depreciation and amortization |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Taxes, other than income taxes |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Total Gas Utility Operating Expenses |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Gas Marketing and other |  |  |  |  |  |  |  |  | 12.8 | 12.6 | 12.6
Total Operating Expenses |  |  |  |  |  |  |  |  | 12.8 | 12.6 | 12.6
Operating Income |  |  |  |  |  |  |  |  | (5.1) | (7.8) | (8.9)
Net Economic Earnings |  |  |  |  |  |  |  |  | (20.7) | (17.6) | (16.3)
Capital Expenditures |  |  |  |  |  |  |  |  | 25.5 | 1.6 | 5.4
Gas Utility: |  |  |  |  |  |  |  |  |  |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues |  |  |  |  |  |  |  |  | 1,660.0 | 1,457.2 | 1,891.8
Gas Utility: | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues |  |  |  |  |  |  |  |  | 1,667.9 | 1,459.4 | 1,895.8
Gas Utility |  |  |  |  |  |  |  |  |  |  |
Natural and propane gas |  |  |  |  |  |  |  |  | 645.9 | 539.7 | 957.6
Other operation and maintenance expenses |  |  |  |  |  |  |  |  | 409.1 | 379.3 | 391.6
Depreciation and amortization |  |  |  |  |  |  |  |  | 153.5 | 136.9 | 129.9
Taxes, other than income taxes |  |  |  |  |  |  |  |  | 137.8 | 125.2 | 142.1
Total Gas Utility Operating Expenses |  |  |  |  |  |  |  |  | 1,346.3 | 1,181.1 | 1,621.2
Gas Marketing and other |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Total Operating Expenses |  |  |  |  |  |  |  |  | 1,346.3 | 1,181.1 | 1,621.2
Operating Income |  |  |  |  |  |  |  |  | 321.6 | 278.3 | 274.6
Net Economic Earnings |  |  |  |  |  |  |  |  | 181.5 | 160.3 | 150.4
Capital Expenditures |  |  |  |  |  |  |  |  | 412.6 | 291.7 | 284.4
ASSETS |  |  |  |  |  |  |  |  |  |  |
Total Assets | 5,551.2 |  |  |  | 5,184.7 |  |  |  | 5,551.2 | 5,184.7 | 4,679.3
Gas Utility: | Eliminations |  |  |  |  |  |  |  |  |  |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues |  |  |  |  |  |  |  |  | 7.9 | 2.2 | 4.0
Gas Marketing: |  |  |  |  |  |  |  |  |  |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues |  |  |  |  |  |  |  |  | 79.3 | 78.5 | 82.9
Gas Marketing: | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues |  |  |  |  |  |  |  |  | 79.3 | 78.5 | 153.4
Gas Utility |  |  |  |  |  |  |  |  |  |  |
Natural and propane gas |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Other operation and maintenance expenses |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Depreciation and amortization |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Taxes, other than income taxes |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Total Gas Utility Operating Expenses |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Gas Marketing and other |  |  |  |  |  |  |  |  | 74.1 | 66.7 | 146.6
Total Operating Expenses |  |  |  |  |  |  |  |  | 74.1 | 66.7 | 146.6
Operating Income |  |  |  |  |  |  |  |  | 5.2 | 11.8 | 6.8
Net Economic Earnings |  |  |  |  |  |  |  |  | 6.8 | 6.4 | 4.2
Capital Expenditures |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0
Depreciation and amortization expense |  |  |  |  |  |  |  |  | 0.1 | 0.1 | 0.3
ASSETS |  |  |  |  |  |  |  |  |  |  |
Total Assets | $ 246.2 |  |  |  | $ 205.0 |  |  |  | 246.2 | 205.0 | 160.6
Gas Marketing: | Eliminations |  |  |  |  |  |  |  |  |  |  |
Operating Segment Information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 70.5

X
- Definition Net economic earnings exclude from reported net income the after-tax impact of net unrealized gains and losses and other timing differences associated with energy-related transactions. Net economic earnings will also exclude, if applicable, the after-tax impact of costs related to acquisition, divestiture, and restructuring activities. + References No definition available. + Details
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