Accumulated Other Comprehensive Income (Loss) - Details about Accumulated Other Comprehensive Income Components (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of unrecognized losses and prior service cost	$ (1)	$ (13)	[1]	$ (119)	$ (82)	[1]
Reclassifications of losses into earnings related to fuel derivative contracts	2,572	1,809	[1]	6,927	5,038	[1]
Reclassification out of Accumulated Other Comprehensive Income | Pension and other postretirement liabilities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of unrecognized losses and prior service cost	15	14		45	41
Reclassification out of Accumulated Other Comprehensive Income | Investments and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassifications of losses into earnings related to fuel derivative contracts	$ 0	$ 0		$ 0	$ 2

[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 I, Item 1 of this report for additional information.