Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Aircraft rentals	$ 41	$ 51	$ 121	$ 158