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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Before-Tax Amount
Amortization of prior service cost included in net periodic pension and postretirement cost	$ 1	$ 0
Amortization of actuarial loss included in net periodic pension and postretirement cost	4	5	$ 15	$ 15
Settlement Charge			6	0
Other comprehensive income – pension benefits	5	5	21	15
Tax Benefit
Amortization of prior service cost included in net periodic pension and postretirement cost	0	0
Amortization of actuarial loss included in net periodic pension and postretirement cost	(2)	(1)	(5)	(4)
Settlement Charge			(2)	0
Other comprehensive income – pension benefits	(2)	(1)	(7)	(4)
Net-of-Tax Amount
Amortization of prior service cost included in net periodic pension and postretirement cost	1	0
Amortization of actuarial loss included in net periodic pension and postretirement cost	2	4	10	11
Settlement Charge			4	0
Other comprehensive income – pension benefits	$ 3	$ 4	$ 14	$ 11