Commitments and Contingencies - Rental Expense for Operating Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Gross rental expense	$ 179	$ 182	$ 199
Less: sublease revenue	(16)	(14)	(16)
Less: Rock-McGraw rent credit	0	(4)	(23)
Net rental expense	$ 163	$ 164	$ 160