Derivatives and Risk Management - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016
Fuel Derivative
Derivative
Expected proceeds from derivative instruments				$ 300	$ 300
Expected payments for derivative instruments									$ 300
Scenario, Forecast | Fuel Derivative
Derivative
Expected payments for derivative instruments								$ 300
Fuel hedge contracts
Derivative
Derivative losses	$ 100		$ 11			$ 3	$ 326
Fuel hedge contracts | Airline
Derivative
Settlement of derivative contract		$ 455