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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Before-Tax Amount
Amortization of prior service cost included in net periodic pension and postretirement cost	$ (1)	$ 0	$ (1)	$ 0
Amortization of actuarial loss included in net periodic pension and postretirement cost	7	5	11	10
Settlement Charge			6	0
Other comprehensive income – pension benefits	6	5	16	10
Tax Benefit
Amortization of prior service cost included in net periodic pension and postretirement cost	0	0	0	0
Amortization of actuarial loss included in net periodic pension and postretirement cost	(2)	(2)	(3)	(3)
Settlement Charge			(2)	0
Other comprehensive income – pension benefits	(2)	(2)	(5)	(3)
Net-of-Tax Amount
Amortization of prior service cost included in net periodic pension and postretirement cost	(1)	0	(1)	0
Amortization of actuarial loss included in net periodic pension and postretirement cost	5	3	8	7
Settlement Charge			4	0
Other comprehensive income – pension benefits	$ 4	$ 3	$ 11	$ 7