Summary of Significant Accounting Policies - Foreign Currency (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Cumulative translation loss, net of tax	$ 582	$ 430
Foreign currency exchange losses, net	$ 76	$ 66	$ 87