Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Aircraft rentals	$ 40	$ 54

Expenses incurred related to the lease of aircraft from outside third parties that are used in the entity's business operations.

No definition available.