Derivatives and Risk Management - Narrative (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative, gain (loss), net					$ 1,366	$ (2,186)	$ 86
Expected payments for (proceeds from) derivative instrument		$ (300)
Proceeds from hedging activities					429	0	0
Payments for derivative instrument					71	0	$ 0
Unrealized gain on foreign currency exchange contracts						150
Cash flow hedge gain to be reclassified within twelve months					75
Hedge margin receivable		$ 119			119	925
Fuel hedge contract
Derivative [Line Items]
Derivative, gain (loss), net					$ (741)	$ (2,000)
Forecast
Derivative [Line Items]
Expected payments for (proceeds from) derivative instrument	$ (300)		$ 300	$ 300
Subsequent Event | Forecast
Derivative [Line Items]
Expected payments for (proceeds from) derivative instrument				$ 725