Lease Commitments - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leased Assets [Line Items]
Rent expense	$ 34	$ 39	$ 45
Minimum
Operating Leased Assets [Line Items]
Lease terms	1 year
Maximum
Operating Leased Assets [Line Items]
Lease terms	15 years