Selected Quarterly Financial Data (Unaudited) - Impact of Significant Items on Quarterly Results (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Impairment of assets	$ 102	$ 0	$ 61	$ 8	$ 232	$ 11	$ 111	$ 23	$ 171	$ 377		$ 25
Severance and benefit costs	2	2	6	6	7	9	11	14	16	41		116
Termination of an engine maintenance service agreement					64	0	0	0	0	64		0
(Gains) losses on sale of assets and other special charges	26	25	4	4	(2)	(3)	7	3	59	5		35
Total operating special charges	130	27	71	18	301	17	129	40	246	487	[1]	176	[1]
Nonoperating unrealized (gains) losses on investments	(81)	(21)	(34)	(17)	(56)	(29)	135	(45)	(153)	5		0
Total special charges and unrealized (gains) losses on investments	49	6	37	1	245	(12)	264	(5)	93	492		176
Income tax benefit related to special charges and unrealized (gains) losses on investments	(11)	(2)	(8)	0	(55)	3	(59)	1	(21)	(110)		(63)
Income tax adjustments					(5)	0	0	0	0	(5)		(189)
Total operating special charges, net of income taxes and income tax adjustments	$ 38	$ 4	$ 29	$ 1	$ 185	$ (9)	$ 205	$ (4)	$ 72	$ 377		$ (76)

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.