Pension Plans, Postretirement and Other Employee Benefits - Amounts Recognized for Pension and Postretirement Benefits in Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Before-Tax Amount
Amortization of actuarial loss included in net periodic pension and postretirement cost	$ 4	$ 4
Settlement charge (a)	0	6
Other comprehensive income – pension benefits	4	10
Tax Benefit
Amortization of actuarial loss included in net periodic pension and postretirement cost	(1)	(1)
Settlement charge (a)	0	(2)
Other comprehensive income – pension benefits	(1)	(3)
Net-of-Tax Amount
Amortization of actuarial loss included in net periodic pension and postretirement cost	3	3
Settlement charge (a)	0	4
Other comprehensive income – pension benefits	$ 3	$ 7