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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Before-Tax Amount
Amortization of prior service cost included in net periodic pension and postretirement cost	$ (1)	$ (1)	$ (1)	$ (1)
Amortization of actuarial loss included in net periodic pension and postretirement cost	6	7	10	11
Settlement charge			0	6
Other comprehensive income (loss) – pension benefits	5	6	9	16
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	(1)	(2)	(2)	(3)
Settlement charge			0	(2)
Other comprehensive income (loss) – pension benefits	(1)	(2)	(2)	(5)
Net-of-Tax Amount
Amortization of prior service cost included in net periodic pension and postretirement cost	(1)	(1)	(1)	(1)
Amortization of actuarial loss included in net periodic pension and postretirement cost	5	5	8	8
Settlement charge			0	4
Other comprehensive income (loss) – pension benefits	$ 4	$ 4	$ 7	$ 11