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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Compensation and Retirement Disclosure [Abstract]
Amortization of prior service cost included in net periodic pension and postretirement cost, Before-tax amount	$ (1)	$ (2)	$ (2)	$ (2)
Amortization of actuarial loss included in net periodic pension and postretirement cost, Before-tax amount	4	5	9	9
Other comprehensive income - pension benefits, Before-tax amount	3	3	7	7
Amortization of prior service cost included in net periodic pension and postretirement cost, Tax Benefit	0	0	0	0
Amortization of actuarial loss included in net periodic pension and postretirement cost, Tax Benefit	(1)	0	(2)	(1)
Other comprehensive - pension benefits, Tax Benefit	(1)	0	(2)	(1)
Amortization of prior service cost included in net periodic pension and postretirement cost, Net-of-tax Amount	(1)	(2)	(2)	(2)
Amortization of actuarial loss included in net periodic pension and postretirement cost, Net-of-tax Amount	3	5	7	8
Other comprehensive income - pension benefits, Net-of-tax Amount	$ 2	$ 3	$ 5	$ 6