Income Taxes - Deferred Tax Assets and Liabilities on Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Deferred income taxes - assets	$ 181	$ 286
Deferred income taxes - liabilities	(63)	(47)
Net deferred tax asset	$ 118	$ 239