Derivatives and Risk Management - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Proceeds from derivative instruments	$ 20	$ 291	$ 429
Payments for derivative instruments	244	451	71
Derivative losses	(7)	(342)	(1,366)
Margin posted due to fair value position of hedge contracts	43	38
Fuel derivative
Derivative [Line Items]
Proceeds from derivative instruments		300	300
Payments for derivative instruments	300	300
Fuel hedge contracts
Derivative [Line Items]
Derivative losses	$ 81	366	$ 741
Fuel hedge contracts | Airline
Derivative [Line Items]
Settlement of derivative contract		$ 455