Commitments and Contingencies - Rental Expense for Operating Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Gross rental expense	$ 177	$ 179	$ 182
Less: sublease revenue	(17)	(16)	(14)
Less: rent credit	0	0	(4)
Net rental expense	$ 160	$ 163	$ 164