Accumulated Other Comprehensive Income (Loss) - Components of the Company's AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, deferred taxes	$ (39)		$ 26		$ (256)
Balance	8,734	[1]	8,574		8,966
Other comprehensive income (loss) before reclassifications, deferred taxes	(83)		74		187
Other comprehensive income (loss) before reclassifications, net of tax	289		(239)		(334)
Amounts reclassified from accumulated other comprehensive income, deferred taxes	(13)		(21)		95
Amounts reclassified from accumulated other comprehensive income, net of tax	49		39		336
Reclassification of stranded tax effects, tax			(118)
Amounts reclassified to retained earnings, deferred taxes	(1)
Amounts reclassified to retained earnings, net of taxes	6
Balance, deferred taxes	(136)		(39)		26
Balance	9,995		8,734	[1]	8,574
Decrease (increase) in prior service credits	3		0		(30)
Decrease (increase) in actuarial losses	380		306		560
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(1,147)		(829)		(831)
Reclassification of stranded tax effects			(118)
Balance	(803)		(1,147)		(829)
Pension and Other Postretirement Liabilities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, before tax	(1,102)		(854)		(363)
Other comprehensive income (loss) before reclassifications, before tax	377		(306)		(517)
Amounts reclassified from accumulated other comprehensive income, before tax	62		58		26
Amounts reclassified to retained earnings, before tax	0
Balance, before tax	(663)		(1,102)		(854)
Fuel Derivatives Contracts
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, before tax	0		(2)		(215)
Other comprehensive income (loss) before reclassifications, before tax	0		0		(4)
Amounts reclassified from accumulated other comprehensive income, before tax	0		2		217
Amounts reclassified to retained earnings, before tax	0
Balance, before tax	0		0		(2)
Investments and Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, before tax	(6)		1		3
Other comprehensive income (loss) before reclassifications, before tax	(5)		(7)		0
Amounts reclassified from accumulated other comprehensive income, before tax	0		0		(2)
Amounts reclassified to retained earnings, before tax	7
Balance, before tax	$ (4)		$ (6)		$ 1

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.