Pension and Other Postretirement Benefits - Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension And Other Postretirement Footnote Text Details [Abstract]
Period decrease in unfunded liability for pension and other postretirement benefit plans	$ 182
Postretirement benefits liability adjustment, (increase) decrease to equity, pre-tax	(131)
Postretirement benefits liability adjustment, (increase) decrease to equity, after-tax	(85)	$ 426	$ (539)
Pension Benefits [Member]
Change in benefit obligation [Abstract]
Benefit obligation, January 1	5,269	4,700
Service cost	2	2	3
Interest cost	194	206	181
(Gain) loss from past experience	(239)	679
Effect of plan ammendment	0	2
Benefits paid from plan assets	(270)	(291)
Benefits paid - other	(22)	(29)
Benefit obligation, December 31	4,934	5,269	4,700
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	4,170	4,089
Actual return on plan assets	75	257
Benefits paid	(270)	(291)
Contributions	6	115
Fair value of plan assets, December 31	3,981	4,170	4,089
Funded Status	(953)	(1,099)
Postretirement benefits liability adjustment included in AOCI table details [Abstract]
Unrecognized net gain (loss)	(2,201)	(2,317)
Unrecognized prior service cost	(7)	(7)
Postretirement benefits liabilty adjustment	(2,208)	(2,324)
Components of net pension cost and other postretirement benefits cost tables detail [Abstract]
Service cost	2	2	3
Interest cost	194	206	181
Expected long-term return on plan assets	(267)	(264)	(272)
Amortization of net loss from past experience	70	57	74
Settlement loss	0	6	0
Defined benefit plan cost	(1)	7	(14)
Pension And Other Postretirement Footnote Text Details [Abstract]
Expected pre-tax gains (losses) related to amortization of past experience	$ (66)
Weighted average amortization period for amortization of past experience	28 years
Other Postretirement Benefits [Member]
Change in benefit obligation [Abstract]
Benefit obligation, January 1	$ 335	323
Service cost	0	0
Interest cost	11	12
(Gain) loss from past experience	(19)	31
Effect of plan ammendment	0	0
Benefits paid from plan assets	(3)	(5)
Benefits paid - other	(29)	(26)
Benefit obligation, December 31	295	335	323
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	12	16
Actual return on plan assets	(1)	1
Benefits paid	(3)	(5)
Contributions	0	0
Fair value of plan assets, December 31	8	12	16
Funded Status	(287)	(323)
Postretirement benefits liability adjustment included in AOCI table details [Abstract]
Unrecognized net gain (loss)	1	(16)
Unrecognized prior service cost	52	54
Postretirement benefits liabilty adjustment	53	38
Components of net pension cost and other postretirement benefits cost tables detail [Abstract]
Service cost	0	0
Interest cost	11	12
Defined benefit plan cost	$ 8	$ 9	(11)
Pension And Other Postretirement Footnote Text Details [Abstract]
Curtailment gain (loss)			$ 19