Pension and Other Postretirement Benefits - Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension And Other Postretirement Footnote Text Details [Abstract]
Postretirement benefits liability adjustment, (increase) decrease to equity, pre-tax			$ (28)
Postretirement benefits liability adjustment, (increase) decrease to equity, after-tax	$ (8)	$ (20)	(19)	$ (31)
Pension Benefits [Member]
Change in benefit obligation [Abstract]
Service cost	1	0	1	1
Interest cost	49	49	99	97
Change in plan assets [Roll Forward]
Contributions			0
Components of net pension cost and other postretirement benefits cost tables detail [Abstract]
Service cost	1	0	1	1
Interest cost	49	49	99	97
Expected long-term return on plan assets	(62)	(67)	(124)	(133)
Amortization of net loss from past experience	16	18	33	35
Amortization of prior service cost	0	0	0	0
Defined benefit plan cost	4	0	9	0
Pension And Other Postretirement Footnote Text Details [Abstract]
Expected pension contributions for the remainder of the year	0		0
Other Postretirement Benefits [Member]
Change in benefit obligation [Abstract]
Service cost	0	0	0	0
Interest cost	2	2	5	5
Components of net pension cost and other postretirement benefits cost tables detail [Abstract]
Service cost	0	0	0	0
Interest cost	2	2	5	5
Expected long-term return on plan assets	0	0	0	0
Amortization of net loss from past experience	0	0	0	0
Amortization of prior service cost	0	0	(1)	(1)
Defined benefit plan cost	$ 2	$ 2	$ 4	$ 4