Commitments and Contingencies - Rental Expense for Operating Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Commitments and Contingencies Disclosure [Abstract]
Gross rental expense	$ 182	$ 199	$ 202
Less: sublease revenue	(14)	(16)	(29)
Less: Rock-McGraw rent credit	(4)	(23)	(20)
Net rental expense	$ 164	$ 160	$ 153