RECENTLY ADOPTED ACCOUNTING PRONOUNCEMENTS (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Jan. 01, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and cash equivalents			$ 177	$ 194	$ 183
Cash, cash equivalents, and restricted cash	$ 197	$ 328	183	197	183
Current Assets
Prepaid expenses and other current assets			174	133
Current Liabilities
Air traffic liability			1,122	806
Other accrued liabilities			433	400
Deferred revenue, current			649	635
Other Liabilities and Credits
Deferred income taxes			372	370
Deferred revenue, noncurrent			1,102	1,090
Other liabilities			423	425
Shareholders' Equity
Retained earnings			$ 4,219	4,193
Operating Revenues
Passenger revenues	1,685	1,602
Mileage plan other revenue	107	100
Cargo and other	40	38
Total Operating Revenues	1,832	1,740
Operating Expenses
Wages and benefits	536	450
Selling expenses	78	83
Special items—merger-related costs	6	39
All other operating expenses		1,011
Total Operating Expenses	1,803	1,583
Operating Income	29	157
Nonoperating Income (Expense)
Other—net	(12)	(1)
All other nonoperating income (expense)		(14)
Nonoperating Income (Expense) Total	(23)	(15)
Income Before Income Tax	6	142
Income tax expense	2	49
Net Income (loss)	4	93
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	94	90
Stock-based compensation and other	8	13
Changes in deferred tax provision	2	44
Increase in air traffic liability	316	364
Increase in deferred revenue	26	31
Other—net	(144)	(165)
Net cash provided by operating activities	306	470
Net cash used in investing activities	(164)	(488)
Net cash used in financing activities	(156)	(127)
Net increase (decrease) in cash, cash equivalents, and restricted cash	(14)	(145)
Cash, cash equivalents, and restricted cash at beginning of year	197	328
Cash, cash equivalents, and restricted cash at end of the period	$ 183	183
As Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash, cash equivalents, and restricted cash		328			183
Current Assets
Prepaid expenses and other current assets				127
Current Liabilities
Air traffic liability				937
Deferred revenue, current				518
Other Liabilities and Credits
Deferred income taxes				454
Deferred revenue, noncurrent				699
Other liabilities				451
Shareholders' Equity
Retained earnings				4,454
Operating Revenues
Passenger revenues		1,484
Mileage plan other revenue		119
Cargo and other		146
Total Operating Revenues		1,749
Operating Expenses
Wages and benefits		448
Selling expenses		81
Special items—merger-related costs		40
All other operating expenses		1,011
Total Operating Expenses		1,580
Operating Income		169
Nonoperating Income (Expense)
Other—net		(3)
All other nonoperating income (expense)		(14)
Nonoperating Income (Expense) Total		(17)
Income Before Income Tax		152
Income tax expense		53
Net Income (loss)		99
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		90
Stock-based compensation and other		13
Changes in deferred tax provision		48
Increase in air traffic liability		369
Increase in deferred revenue		1
Other—net		(150)
Net cash provided by operating activities		470
Net cash used in investing activities		(488)
Net cash used in financing activities		(127)
Net increase (decrease) in cash, cash equivalents, and restricted cash		(145)
Cash, cash equivalents, and restricted cash at beginning of year		328
Cash, cash equivalents, and restricted cash at end of the period		183
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
Current Assets
Prepaid expenses and other current assets				6
Current Liabilities
Air traffic liability				(131)
Deferred revenue, current				117
Other Liabilities and Credits
Deferred income taxes				(84)
Deferred revenue, noncurrent				391
Other liabilities				(26)
Shareholders' Equity
Retained earnings				$ (261)
Operating Revenues
Passenger revenues		118
Mileage plan other revenue		(19)
Cargo and other		(108)
Total Operating Revenues		(9)
Operating Expenses
Wages and benefits		0
Selling expenses		2
Special items—merger-related costs		(1)
All other operating expenses		0
Total Operating Expenses		1
Operating Income		(10)
Nonoperating Income (Expense)
Other—net		0
All other nonoperating income (expense)		0
Nonoperating Income (Expense) Total		0
Income Before Income Tax		(10)
Income tax expense		(4)
Net Income (loss)		(6)
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		(4)
Increase in air traffic liability		(5)
Increase in deferred revenue		30
Other—net		(15)
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
Accounting Standards Update 2017-07 [Member] | Adjustments [Member]
Operating Revenues
Passenger revenues		0
Mileage plan other revenue		0
Cargo and other		0
Total Operating Revenues		0
Operating Expenses
Wages and benefits		2
Selling expenses		0
Special items—merger-related costs		0
All other operating expenses		0
Total Operating Expenses		2
Operating Income		(2)
Nonoperating Income (Expense)
Other—net		2
All other nonoperating income (expense)		0
Nonoperating Income (Expense) Total		2
Income Before Income Tax		0
Income tax expense		0
Net Income (loss)		$ 0