New Accounting Pronouncements (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) aircraft	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) aircraft	Jun. 30, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Aircraft rentals	$ 40	$ 53	$ 80	$ 107
Aircraft under operating lease | aircraft	53		53
Net Present Value Aircraft Operating Leases	$ 850		$ 850
ASU 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	4		$ 6
AOCI Including Portion Attributable to Noncontrolling Interest
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 20
B-717-200
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Aircraft under operating lease | aircraft	76		76