Pension and Other Postretirement Employee Benefit Plans - Changes In Level 3 Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
U.S. defined benefit pension plans
Change in plan assets
Fair value of plan assets, end of year	$ 14,623	$ 14,354	$ 13,012
U.S. defined benefit pension plans | Equity securities
Change in plan assets
Fair value of plan assets, end of year	5,310	5,449
U.S. defined benefit pension plans | Common/collective trust funds
Change in plan assets
Fair value of plan assets, end of year	1,630	1,312
U.S. defined benefit pension plans | Total limited partnerships
Change in plan assets
Fair value of plan assets, end of year	3,116	3,871
U.S. defined benefit pension plans | Hedge funds
Change in plan assets
Fair value of plan assets, end of year	103	1,073
U.S. defined benefit pension plans | Private equity
Change in plan assets
Fair value of plan assets, end of year	2,208	1,969
U.S. defined benefit pension plans | Real estate
Change in plan assets
Fair value of plan assets, end of year	533	558
U.S. defined benefit pension plans | Real assets
Change in plan assets
Fair value of plan assets, end of year	272	271
U.S. defined benefit pension plans | Corporate debt securities
Change in plan assets
Fair value of plan assets, end of year	1,463	1,230
U.S. defined benefit pension plans | Mortgage-backed securities
Change in plan assets
Fair value of plan assets, end of year	75	87
U.S. defined benefit pension plans | Other
Change in plan assets
Fair value of plan assets, end of year	2,395	1,685
OPEB Plans
Change in plan assets
Fair value of plan assets, end of year	1,903	1,757	1,563
Level 3 | U.S. defined benefit pension plans
Change in plan assets
Fair value of plan assets, beginning of year	3,961	4,311	3,608
Realized gains/(losses)	439	329	78
Unrealized gains/(losses)	(67)	218	150
Purchases, sales and settlements, net	(960)	(891)	471
Transfers in and/or out of level 3	6	(6)	4
Fair value of plan assets, end of year	3,379	3,961	4,311
Level 3 | U.S. defined benefit pension plans | Equity securities
Change in plan assets
Fair value of plan assets, beginning of year	4	4	1
Realized gains/(losses)	0	0	0
Unrealized gains/(losses)	0	0	(1)
Purchases, sales and settlements, net	0	0	0
Transfers in and/or out of level 3	0	0	4
Fair value of plan assets, end of year	4	4	4
Level 3 | U.S. defined benefit pension plans | Common/collective trust funds
Change in plan assets
Fair value of plan assets, beginning of year	4	199	202
Realized gains/(losses)	0	59	2
Unrealized gains/(losses)	1	(32)	22
Purchases, sales and settlements, net	3	(222)	(27)
Transfers in and/or out of level 3	0	0	0
Fair value of plan assets, end of year	8	4	199
Level 3 | U.S. defined benefit pension plans | Total limited partnerships
Change in plan assets
Fair value of plan assets, beginning of year	3,516	3,687	2,976
Realized gains/(losses)	441	270	76
Unrealized gains/(losses)	23	240	136
Purchases, sales and settlements, net	(968)	(669)	499
Transfers in and/or out of level 3	8	(12)	0
Fair value of plan assets, end of year	3,020	3,516	3,687
Level 3 | U.S. defined benefit pension plans | Hedge funds
Change in plan assets
Fair value of plan assets, beginning of year	718	1,166	1,039
Realized gains/(losses)	193	137	1
Unrealized gains/(losses)	(180)	14	71
Purchases, sales and settlements, net	(662)	(593)	55
Transfers in and/or out of level 3	8	(6)	0
Fair value of plan assets, end of year	77	718	1,166
Level 3 | U.S. defined benefit pension plans | Private equity
Change in plan assets
Fair value of plan assets, beginning of year	1,969	1,743	1,367
Realized gains/(losses)	192	108	59
Unrealized gains/(losses)	173	170	54
Purchases, sales and settlements, net	(126)	(4)	263
Transfers in and/or out of level 3	0	(48)	0
Fair value of plan assets, end of year	2,208	1,969	1,743
Level 3 | U.S. defined benefit pension plans | Real estate
Change in plan assets
Fair value of plan assets, beginning of year	558	467	306
Realized gains/(losses)	29	21	16
Unrealized gains/(losses)	36	44	1
Purchases, sales and settlements, net	(90)	26	144
Transfers in and/or out of level 3	0	0	0
Fair value of plan assets, end of year	533	558	467
Level 3 | U.S. defined benefit pension plans | Real assets
Change in plan assets
Fair value of plan assets, beginning of year	271	311	264
Realized gains/(losses)	27	4	0
Unrealized gains/(losses)	(6)	12	10
Purchases, sales and settlements, net	(90)	(98)	37
Transfers in and/or out of level 3	0	42	0
Fair value of plan assets, end of year	202	271	311
Level 3 | U.S. defined benefit pension plans | Corporate debt securities
Change in plan assets
Fair value of plan assets, beginning of year	7	1	2
Realized gains/(losses)	(2)	0	0
Unrealized gains/(losses)	2	0	0
Purchases, sales and settlements, net	4	0	(1)
Transfers in and/or out of level 3	(2)	6	0
Fair value of plan assets, end of year	9	7	1
Level 3 | U.S. defined benefit pension plans | Mortgage-backed securities
Change in plan assets
Fair value of plan assets, beginning of year	0
Realized gains/(losses)	0
Unrealized gains/(losses)	0
Purchases, sales and settlements, net	1
Transfers in and/or out of level 3	0
Fair value of plan assets, end of year	1
Level 3 | U.S. defined benefit pension plans | Other
Change in plan assets
Fair value of plan assets, beginning of year	430	420	427
Realized gains/(losses)	0	0	0
Unrealized gains/(losses)	(93)	10	(7)
Purchases, sales and settlements, net	0	0	0
Transfers in and/or out of level 3	0	0	0
Fair value of plan assets, end of year	337	430	420
Level 3 | OPEB Plans
Change in plan assets
Fair value of plan assets, beginning of year	1,749	1,554	1,427
Realized gains/(losses)	0	0	0
Unrealized gains/(losses)	154	195	127
Purchases, sales and settlements, net	0	0	0
Transfers in and/or out of level 3	0	0	0
Fair value of plan assets, end of year	$ 1,903	$ 1,749	$ 1,554