Pension and Other Benefit Programs - Components of Pension Plan Expense (Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Interest cost	$ 27	$ 27	$ 34
Estimated return on plan assets	(44)	(44)	(46)
Amortization of loss	2	1	2
Aggregate pension benefit	$ (15)	$ (16)	$ (10)