CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Cash flows from operating activities:
Net Income (Loss)	$ 194	$ (131)	$ (431)	$ (232)	$ 460	$ (877)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	99		105		294	320
Stock-based compensation and other					35	14
Special items - Impairment charges and other	(9)		121		5	350
Restructuring Charges	0		322		(12)	322	$ 220
Changes in certain assets and liabilities:
Changes in deferred tax provision					95	(220)
Increase (Decrease) in Contract with Customer, Liability					152	171
Increase in deferred revenue					73	193
Payment for Pension and Other Postretirement Benefits					(100)	0
Other - net					(101)	(157)
Net cash provided by operating activities					901	116
Property and equipment additions:
Aircraft and aircraft purchase deposits					(52)	(61)
Other flight equipment					(78)	(49)
Other property and equipment					(60)	(94)
Total property and equipment additions, including capitalized interest					(190)	(204)
Purchases of marketable securities					(3,413)	(2,092)
Sales and maturities of marketable securities					2,669	1,520
Other investing activities					(9)	9
Net cash used in investing activities					(943)	(767)
Cash flows from financing activities:
Proceeds from issuance of debt					363	2,581
Common stock repurchases					0	(31)
Dividends paid					0	(45)
Long-term debt payments					(1,222)	(238)
Other financing activities					34	19
Net cash provided by (used in) financing activities					(825)	2,286
Net increase in cash, cash equivalents, and restricted cash					(867)	1,635
Cash, cash equivalents, and restricted cash at beginning of year		$ 1,386		$ 232	1,386	232	232
Cash, cash equivalents, and restricted cash at end of the period	519		1,867		519	1,867	1,386
Cash paid during the period for:
Interest (net of amount capitalized)					100	38
Income taxes						0
Non-cash transactions:
Right-of-use assets acquired through operating leases					126	54
Reconciliation of cash, cash equivalents, and restricted cash at end of the period
Cash and cash equivalents	495		1,855		495	1,855	1,370
Restricted cash included in Prepaid expenses, assets held-for-sale and other current assets	24		12		24	12
Total cash, cash equivalents, and restricted cash at end of the period	$ 519		$ 1,867		$ 519	$ 1,867	$ 1,386