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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Before-Tax Amount
Amortization of prior service cost included in net periodic pension and postretirement costs	$ 0	$ 1	$ (1)	$ 0
Amortization of actuarial loss included in net periodic pension and postretirement costs	5	4	15	15
Settlement charge			0	6
Other comprehensive income – pension benefits	5	5	14	21
Tax Benefit
Amortization of prior service cost included in net periodic pension and postretirement costs	0	0	0	0
Amortization of actuarial loss included in net periodic pension and postretirement costs	(1)	(2)	(3)	(5)
Settlement charge			0	(2)
Other comprehensive income – pension benefits	(1)	(2)	(3)	(7)
Net-of-Tax Amount
Amortization of prior service cost included in net periodic pension and postretirement costs	0	1	(1)	0
Amortization of actuarial loss included in net periodic pension and postretirement costs	4	2	12	10
Settlement charge			0	4
Other comprehensive income – pension benefits	$ 4	$ 3	$ 11	$ 14