Pension and Other Postretirement Benefits - Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension And Other Postretirement Footnote Text Details [Abstract]
Postretirement benefits liability adjustment, (increase) decrease to equity, pre-tax			$ (41)
Postretirement benefits liability adjustment, (increase) decrease to equity, after-tax	$ (9)	$ (11)	(28)	$ (42)
Pension Benefits [Member]
Change in benefit obligation [Abstract]
Service cost	0	1	1	2
Interest cost	50	49	149	146
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	1	2
Interest cost	50	49	149	146
Expected long-term return on plan assets	(62)	(66)	(186)	(199)
Amortization of net loss from past experience	17	17	50	52
Amortization of prior service cost	0	0	0	0
Defined benefit plan cost	5	1	14	1
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	3	3	8	8
Components of net pension cost and other postretirement benefits cost tables detail [Abstract]
Service cost	0	0	0	0
Interest cost	3	3	8	8
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	(1)	(1)	(2)	(2)
Defined benefit plan cost	$ 2	$ 2	$ 6	$ 6