Pension and Other Postretirement Benefits - Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension And Other Postretirement Footnote Text Details [Abstract]
Postretirement benefits liability adjustment, (increase) decrease to equity, pre-tax	$ (16)
Postretirement benefits liability adjustment, (increase) decrease to equity, after-tax	(11)	$ (11)
Pension Benefits [Member]
Change in benefit obligation [Abstract]
Service cost	0	1
Interest cost	50	48
Change in plan assets [Roll Forward]
Contributions	0
Components of net pension cost and other postretirement benefits cost tables detail [Abstract]
Service cost	0	1
Interest cost	50	48
Expected long-term return on plan assets	(62)	(66)
Amortization of net loss from past experience	17	17
Amortization of prior service cost	0	0
Defined benefit plan cost	5	0
Pension And Other Postretirement Footnote Text Details [Abstract]
Expected pension contributions for the remainder of the year	0
Other Postretirement Benefits [Member]
Change in benefit obligation [Abstract]
Service cost	0	0
Interest cost	3	3
Components of net pension cost and other postretirement benefits cost tables detail [Abstract]
Service cost	0	0
Interest cost	3	3
Expected long-term return on plan assets	0	0
Amortization of net loss from past experience	0	0
Amortization of prior service cost	(1)	(1)
Defined benefit plan cost	$ 2	$ 2