RECENTLY ADOPTED ACCOUNTING PRONOUNCEMENTS (Details) - USD ($) $ in Millions			12 Months Ended
		Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total Operating Revenues			$ 8,264	$ 7,894	$ 5,925
Wages and benefits			2,190	1,931	1,394
Prepaid expenses and other current assets			125	133
Air traffic liability			788	806
Deferred Revenue, Current			705	635
Deferred income taxes			512	370
Deferred revenue			1,169	1,090
Other liabilities			418	425
Retained earnings			4,534	4,193
Selling expense			326	368	248
Special items - merger-related costs			87	116	117
Operating Expenses, Other				4,271	2,860
Operating Expenses			7,621	6,686	4,619
Operating Income (Loss)			643	1,208	1,306
Other—net			(23)	3	13
Nonoperating Income (Expense), Other				(52)	(3)
Nonoperating Income (Expense) Total			(58)	(49)	10
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest				1,159	1,316
Income Tax Expense (Benefit)			148	199	519
Net Income			437	960	797
Depreciation and amortization			398	372	363
Stock-based compensation and other			47	55	26
Increase (Decrease) in Deferred Income Taxes			146	45	82
(Increase) decrease in accounts receivable			(25)	(39)	(46)
Increase (decrease) in air traffic liability			(18)	45	3
Increase (decrease) in deferred revenue			149	191	153
Increase (Decrease) in Obligation, Pension and Other Postretirement Benefits				17	23
Other—net			9	(56)	(15)
Net Cash Provided by (Used in) Operating Activities			1,195	1,590	1,386
Net Cash Provided by (Used in) Investing Activities			(631)	(1,129)	(2,622)
Net Cash Provided by (Used in) Financing Activities			(647)	(592)	1,491
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease)				(131)	255
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents			114	197	328		$ 73
Increase in retained earnings due to reclassification of tax effects in AOCI	[1]					$ (170)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect		$ 0
Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total Operating Revenues				7,933	5,931
Wages and benefits				1,924	1,382
Prepaid expenses and other current assets				127
Air traffic liability				937
Deferred Revenue, Current				518
Deferred income taxes				454
Deferred revenue				699
Other liabilities				451
Retained earnings				4,454
Selling expense				357	225
Special items - merger-related costs				118	117
Operating Expenses, Other				4,271	2,860
Operating Expenses				6,670	4,584
Operating Income (Loss)				1,263	1,347
Other—net				(4)	1
Nonoperating Income (Expense), Other				(52)	(3)
Nonoperating Income (Expense) Total				(56)	(2)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest				1,207	1,345
Income Tax Expense (Benefit)				173	531
Net Income				1,034	814
Depreciation and amortization				372	363
Stock-based compensation and other				55	26
Increase (Decrease) in Deferred Income Taxes				(19)	(94)
(Increase) decrease in accounts receivable				39	46
Increase (decrease) in air traffic liability				88	9
Increase (decrease) in deferred revenue				63	83
Increase (Decrease) in Obligation, Pension and Other Postretirement Benefits				17	23
Other—net				(19)	20
Net Cash Provided by (Used in) Operating Activities				1,590	1,386
Net Cash Provided by (Used in) Investing Activities				(1,132)	(2,622)
Net Cash Provided by (Used in) Financing Activities				(592)	1,491
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease)				(134)	255
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents				194	328		73
Accounting Standards Update 2016-18 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents			3
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings due to reclassification of tax effects in AOCI						(170)
Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total Operating Revenues				(39)	(6)
Wages and benefits				0	0
Prepaid expenses and other current assets				6
Air traffic liability				(131)
Deferred Revenue, Current				117
Deferred income taxes				(84)
Deferred revenue				391
Other liabilities				(26)
Retained earnings				(261)
Selling expense				11	23
Special items - merger-related costs				(2)	0
Operating Expenses, Other				0	0
Operating Expenses				9	23
Operating Income (Loss)				(48)	(29)
Other—net				0	0
Nonoperating Income (Expense), Other				0	0
Nonoperating Income (Expense) Total				0	0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest				(48)	(29)
Income Tax Expense (Benefit)				26	(12)
Net Income				(74)	(17)
Depreciation and amortization				0	0
Stock-based compensation and other				0	0
Increase (Decrease) in Deferred Income Taxes				(26)	12
(Increase) decrease in accounts receivable				0	0
Increase (decrease) in air traffic liability				(43)	(6)
Increase (decrease) in deferred revenue				128	70
Increase (Decrease) in Obligation, Pension and Other Postretirement Benefits				0	0
Other—net				(37)	(35)
Net Cash Provided by (Used in) Operating Activities				0	0
Net Cash Provided by (Used in) Investing Activities				3	0
Net Cash Provided by (Used in) Financing Activities				0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease)				3	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents				3	0		$ 0
Accounting Standards Update 2017-07 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total Operating Revenues				0	0
Wages and benefits				7	12
Selling expense				0	0
Special items - merger-related costs				0	0
Operating Expenses, Other				0	0
Operating Expenses				7	12
Operating Income (Loss)				(7)	(12)
Other—net				7	12
Nonoperating Income (Expense), Other				0	0
Nonoperating Income (Expense) Total				7	12
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest				0	0
Income Tax Expense (Benefit)				0	0
Net Income				0	0
Passenger [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				7,301	5,392
Passenger [Member] | Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				6,818	5,006
Passenger [Member] | Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				483	386
Passenger [Member] | Accounting Standards Update 2017-07 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				0	0
Mileage Plan Services, Other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax			434	418	370
Mileage Plan Services, Other [Member] | Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				482	429
Mileage Plan Services, Other [Member] | Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				(64)	(59)
Mileage Plan Services, Other [Member] | Accounting Standards Update 2017-07 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				0	0
Cargo and Freight [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax			198	175	163
Cargo and Freight [Member] | Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				633	496
Cargo and Freight [Member] | Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				(458)	(333)
Cargo and Freight [Member] | Accounting Standards Update 2017-07 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				0	0
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income			$ 437	$ 960	$ 797
Increase in retained earnings due to reclassification of tax effects in AOCI	[1]					$ (170)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect		62
Accumulated Other Comprehensive Loss
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect		$ (62)

[1] Represents the opening balance sheet adjustment recorded as a result of the adoption of the new revenue recognition standard.