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RECENTLY ADOPTED ACCOUNTING PRONOUNCEMENTS (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Dec. 31, 2017 | Jun. 30, 2017 | Jan. 01, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents |  |  |  |  | $ 102 | $ 194 | $ 198 |
Cash, cash equivalents, and restricted cash | $ 108 | $ 198 | $ 197 | $ 328 | 108 | 197 | 198 |
Current Assets |  |  |  |  |  |  |  |
Prepaid expenses and other current assets |  |  |  |  | 180 | 133 |  |
Current Liabilities |  |  |  |  |  |  |  |
Air traffic liability |  |  |  |  | 1,112 | 806 |  |
Other accrued liabilities |  |  |  |  | 483 | 400 |  |
Deferred revenue, current |  |  |  |  | 667 | 635 |  |
Other Liabilities and Credits |  |  |  |  |  |  |  |
Deferred income taxes |  |  |  |  | 437 | 370 |  |
Deferred revenue, noncurrent |  |  |  |  | 1,110 | 1,090 |  |
Other liabilities |  |  |  |  | 417 | 425 |  |
Shareholders' Equity |  |  |  |  |  |  |  |
Retained earnings |  |  |  |  | $ 4,373 | $ 4,193 |  |
Operating Revenues |  |  |  |  |  |  |  |
Total Operating Revenues | 2,156 | 2,102 | 3,988 | 3,842 |  |  |  |
Operating Expenses |  |  |  |  |  |  |  |
Wages and benefits | 544 | 470 | 1,080 | 920 |  |  |  |
Selling expenses | 88 | 102 | 166 | 185 |  |  |  |
Special items—merger-related costs | 39 | 24 | 45 | 63 |  |  |  |
All other operating expenses |  | 1,019 |  | 2,030 |  |  |  |
Total Operating Expenses | 1,885 | 1,615 | 3,688 | 3,198 |  |  |  |
Operating Income | 271 | 487 | 300 | 644 |  |  |  |
Nonoperating Income (Expense) |  |  |  |  |  |  |  |
Other—net | (1) | 0 | (13) | (1) |  |  |  |
Nonoperating Income (Expense), Other |  | (13) |  | (27) |  |  |  |
Nonoperating Income (Expense) Total | (12) | (13) | (35) | (28) |  |  |  |
Income Before Income Tax | 259 | 474 | 265 | 616 |  |  |  |
Income tax expense | 66 | 181 | 68 | 230 |  |  |  |
Net Income (loss) | 193 | 293 | 197 | 386 |  |  |  |
Adjustments to reconcile net income to net cash provided by operating activities: |  |  |  |  |  |  |  |
Depreciation and amortization | 97 | 90 | 191 | 180 |  |  |  |
Stock-based compensation and other |  |  | 17 | 25 |  |  |  |
Changes in deferred tax provision |  |  | 67 | 127 |  |  |  |
Increase in air traffic liability |  |  | 306 | 384 |  |  |  |
Increase in deferred revenue |  |  | 52 | 69 |  |  |  |
Other—net |  |  | (104) | (87) |  |  |  |
Net cash provided by operating activities |  |  | 726 | 1,084 |  |  |  |
Net cash used in investing activities |  |  | (468) | (971) |  |  |  |
Net cash used in financing activities |  |  | (347) | (243) |  |  |  |
Net increase (decrease) in cash, cash equivalents, and restricted cash |  |  | (89) | (130) |  |  |  |
Cash, cash equivalents, and restricted cash at beginning of year |  |  | 197 | 328 |  |  |  |
Cash, cash equivalents, and restricted cash at end of the period | 108 | 198 | 108 | 198 |  |  |  |
As Reported [Member] |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Cash, cash equivalents, and restricted cash |  | 198 |  | 328 |  |  | 198 |
Operating Revenues |  |  |  |  |  |  |  |
Mileage plan other revenue |  | 128 |  | 247 |  |  |  |
Total Operating Revenues |  | 2,102 |  | 3,851 |  |  |  |
Operating Expenses |  |  |  |  |  |  |  |
Wages and benefits |  | 469 |  | 917 |  |  |  |
Selling expenses |  | 97 |  | 178 |  |  |  |
Special items—merger-related costs |  | 24 |  | 64 |  |  |  |
All other operating expenses |  | 1,019 |  | 2,030 |  |  |  |
Total Operating Expenses |  | 1,609 |  | 3,189 |  |  |  |
Operating Income |  | 493 |  | 662 |  |  |  |
Nonoperating Income (Expense) |  |  |  |  |  |  |  |
Other—net |  | (1) |  | (4) |  |  |  |
Nonoperating Income (Expense), Other |  | (13) |  | (27) |  |  |  |
Nonoperating Income (Expense) Total |  | (14) |  | (31) |  |  |  |
Income Before Income Tax |  | 479 |  | 631 |  |  |  |
Income tax expense |  | 183 |  | 236 |  |  |  |
Net Income (loss) |  | 296 |  | 395 |  |  |  |
Adjustments to reconcile net income to net cash provided by operating activities: |  |  |  |  |  |  |  |
Depreciation and amortization |  |  |  | 180 |  |  |  |
Stock-based compensation and other |  |  |  | 25 |  |  |  |
Changes in deferred tax provision |  |  |  | 132 |  |  |  |
Increase in air traffic liability |  |  |  | 406 |  |  |  |
Increase in deferred revenue |  |  |  | 15 |  |  |  |
Other—net |  |  |  | (69) |  |  |  |
Net cash provided by operating activities |  |  |  | 1,084 |  |  |  |
Net cash used in investing activities |  |  |  | (971) |  |  |  |
Net cash used in financing activities |  |  |  | (243) |  |  |  |
Net increase (decrease) in cash, cash equivalents, and restricted cash |  |  |  | (130) |  |  |  |
Cash, cash equivalents, and restricted cash at beginning of year |  |  |  | 328 |  |  |  |
Cash, cash equivalents, and restricted cash at end of the period |  | 198 |  | 198 |  |  |  |
Accounting Standards Update 2014-09 [Member] |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Cumulative effect of new accounting pronouncements |  |  |  |  |  |  |  | $ 171
Accounting Standards Update 2014-09 [Member] | Adjustments [Member] |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Cash, cash equivalents, and restricted cash |  | 0 |  | 0 |  |  | $ 0 |
Operating Revenues |  |  |  |  |  |  |  |
Mileage plan other revenue |  | (19) |  | (38) |  |  |  |
Total Operating Revenues |  | 0 |  | (9) |  |  |  |
Operating Expenses |  |  |  |  |  |  |  |
Wages and benefits |  | 0 |  | 0 |  |  |  |
Selling expenses |  | 5 |  | 7 |  |  |  |
Special items—merger-related costs |  | 0 |  | (1) |  |  |  |
All other operating expenses |  | 0 |  | 0 |  |  |  |
Total Operating Expenses |  | 5 |  | 6 |  |  |  |
Operating Income |  | (5) |  | (15) |  |  |  |
Nonoperating Income (Expense) |  |  |  |  |  |  |  |
Other—net |  | 0 |  | 0 |  |  |  |
Nonoperating Income (Expense), Other |  | 0 |  | 0 |  |  |  |
Nonoperating Income (Expense) Total |  | 0 |  | 0 |  |  |  |
Income Before Income Tax |  | (5) |  | (15) |  |  |  |
Income tax expense |  | (2) |  | (6) |  |  |  |
Net Income (loss) |  | (3) |  | (9) |  |  |  |
Adjustments to reconcile net income to net cash provided by operating activities: |  |  |  |  |  |  |  |
Depreciation and amortization |  |  |  | 0 |  |  |  |
Stock-based compensation and other |  |  |  | 0 |  |  |  |
Changes in deferred tax provision |  |  |  | (5) |  |  |  |
Increase in air traffic liability |  |  |  | (22) |  |  |  |
Increase in deferred revenue |  |  |  | 54 |  |  |  |
Other—net |  |  |  | (18) |  |  |  |
Net cash provided by operating activities |  |  |  | 0 |  |  |  |
Net cash used in investing activities |  |  |  | 0 |  |  |  |
Net cash used in financing activities |  |  |  | 0 |  |  |  |
Net increase (decrease) in cash, cash equivalents, and restricted cash |  |  |  | 0 |  |  |  |
Cash, cash equivalents, and restricted cash at beginning of year |  |  |  | 0 |  |  |  |
Cash, cash equivalents, and restricted cash at end of the period |  | 0 |  | 0 |  |  |  |
Accounting Standards Update 2017-07 [Member] | Adjustments [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Mileage plan other revenue |  | 0 |  | 0 |  |  |  |
Total Operating Revenues |  | 0 |  | 0 |  |  |  |
Operating Expenses |  |  |  |  |  |  |  |
Wages and benefits |  | 1 |  | 3 |  |  |  |
Selling expenses |  | 0 |  | 0 |  |  |  |
Special items—merger-related costs |  | 0 |  | 0 |  |  |  |
All other operating expenses |  | 0 |  | 0 |  |  |  |
Total Operating Expenses |  | 1 |  | 3 |  |  |  |
Operating Income |  | (1) |  | (3) |  |  |  |
Nonoperating Income (Expense) |  |  |  |  |  |  |  |
Other—net |  | 1 |  | 3 |  |  |  |
Nonoperating Income (Expense), Other |  | 0 |  | 0 |  |  |  |
Nonoperating Income (Expense) Total |  | 1 |  | 3 |  |  |  |
Income Before Income Tax |  | 0 |  | 0 |  |  |  |
Income tax expense |  | 0 |  | 0 |  |  |  |
Net Income (loss) |  | 0 |  | 0 |  |  |  |
Cargo and Freight [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Revenue from contract with customer, including assessed taxes | 51 | 48 | 91 | 86 |  |  |  |
Cargo and Freight [Member] | As Reported [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Revenue from contract with customer, including assessed taxes |  | 167 |  | 313 |  |  |  |
Cargo and Freight [Member] | Accounting Standards Update 2014-09 [Member] | Adjustments [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Revenue from contract with customer, including assessed taxes |  | (119) |  | (227) |  |  |  |
Cargo and Freight [Member] | Accounting Standards Update 2017-07 [Member] | Adjustments [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Revenue from contract with customer, including assessed taxes |  | 0 |  | 0 |  |  |  |
Passenger [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Revenue from contract with customer, including assessed taxes | 1,997 | 1,945 | 3,682 | 3,547 |  |  |  |
Passenger [Member] | As Reported [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Revenue from contract with customer, including assessed taxes |  | 1,807 |  | 3,291 |  |  |  |
Passenger [Member] | Accounting Standards Update 2014-09 [Member] | Adjustments [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Revenue from contract with customer, including assessed taxes |  | 138 |  | 256 |  |  |  |
Passenger [Member] | Accounting Standards Update 2017-07 [Member] | Adjustments [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Revenue from contract with customer, including assessed taxes |  | 0 |  | 0 |  |  |  |
Cargo and Other Revenue [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 51 | 48 | 91 | 86 |  |  |  |
Cargo and Other Revenue [Member] | Cargo and Freight [Member] |  |  |  |  |  |  |  |
Operating Revenues |  |  |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | $ 51 | $ 48 | $ 91 | $ 86 |  |  |  |

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