Selected Quarterly Financial Data (Unaudited) - Impact of Significant Items on Quarterly Results (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Operating:
Impairment of assets	$ 232	$ 11	$ 111	$ 23	$ 10	$ 15	$ 0	$ 0	$ 377	$ 25		$ 412
Termination of an engine maintenance service agreement	64	0	0	0					64	0		0
Severance and benefit costs	7	9	11	14	15	23	41	37	41	116		37
(Gains) losses on sale of assets and other special charges	(2)	(3)	7	3	6	12	3	14	5	35		51
Total operating special charges	301	17	129	40	31	50	44	51	487	176	[1]	745	[1]
Nonoperating income (expense):
Nonoperating mark-to-market (MTM) (gains) losses on financial instruments	(56)	(29)	135	(45)					5	0		0
Total special charges and MTM (gains) losses on financial instruments	245	(12)	264	(5)					492	176		637
Income taxes:
Income tax benefit related to special charges	(55)	3	(59)	1	(11)	(18)	(16)	(18)	(110)	(63)		(229)
Income tax adjustments	(5)	0	0	0	(179)	0	0	0	(5)	(179)		180
Total operating special charges, net of income taxes and income tax adjustments	$ 185	$ (9)	$ 205	$ (4)	$ (159)	$ 32	$ 28	$ 33	$ 377	$ (66)		$ 588

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) and Accounting Standards Update No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.