Pension and Other Postretirement Employee Benefit Plans - Changes In Level 3 Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
U.S. defined benefit pension plans
Change in plan assets
Fair value of plan assets, beginning of year	$ 14,623	$ 14,354
Fair value of plan assets, end of year	14,125	14,623	$ 14,354
U.S. defined benefit pension plans \| Equity securities
Change in plan assets
Fair value of plan assets, beginning of year	5,310
Fair value of plan assets, end of year	4,833	5,310
U.S. defined benefit pension plans \| Corporate debt securities
Change in plan assets
Fair value of plan assets, beginning of year	1,463
Fair value of plan assets, end of year	1,621	1,463
U.S. defined benefit pension plans \| Mortgage-backed securities
Change in plan assets
Fair value of plan assets, beginning of year	75
Fair value of plan assets, end of year	68	75
U.S. defined benefit pension plans \| Other
Change in plan assets
Fair value of plan assets, beginning of year	2,395
Fair value of plan assets, end of year	2,321	2,395
OPEB plans
Change in plan assets
Fair value of plan assets, beginning of year	1,903	1,757
Fair value of plan assets, end of year	1,855	1,903	1,757
Level 3 \| U.S. defined benefit pension plans
Change in plan assets
Fair value of plan assets, beginning of year	351	441	425
Actual return on plan assets, Realized gains/(losses)	0	(2)	0
Actual return on plan assets, Unrealized gains/(losses)	195	(91)	10
Purchases, sales and settlements, net	(7)	5	0
Transfers in and/or out of level 3	0	(2)	6
Fair value of plan assets, end of year	539	351	441
Level 3 \| U.S. defined benefit pension plans \| Equity securities
Change in plan assets
Fair value of plan assets, beginning of year	4	4	4
Actual return on plan assets, Realized gains/(losses)	0	0	0
Actual return on plan assets, Unrealized gains/(losses)	(2)	0	0
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	2	4	4
Level 3 \| U.S. defined benefit pension plans \| Corporate debt securities
Change in plan assets
Fair value of plan assets, beginning of year	9	7	1
Actual return on plan assets, Realized gains/(losses)	0	(2)	0
Actual return on plan assets, Unrealized gains/(losses)	0	2	0
Purchases, sales and settlements, net	(7)	4	0
Transfers in and/or out of level 3	0	(2)	6
Fair value of plan assets, end of year	2	9	7
Level 3 \| U.S. defined benefit pension plans \| Mortgage-backed securities
Change in plan assets
Fair value of plan assets, beginning of year	1	0	0
Actual return on plan assets, Realized gains/(losses)	0	0	0
Actual return on plan assets, Unrealized gains/(losses)	0	0	0
Purchases, sales and settlements, net	0	1	0
Transfers in and/or out of level 3	0	0	0
Fair value of plan assets, end of year	1	1	0
Level 3 \| U.S. defined benefit pension plans \| Other
Change in plan assets
Fair value of plan assets, beginning of year	337	430	420
Actual return on plan assets, Realized gains/(losses)	0	0	0
Actual return on plan assets, Unrealized gains/(losses)	197	(93)	10
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	534	337	430
Level 3 \| OPEB plans
Change in plan assets
Fair value of plan assets, beginning of year	1,903	1,749	1,554
Actual return on plan assets, Realized gains/(losses)	0	0	0
Actual return on plan assets, Unrealized gains/(losses)	(48)	154	195
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	$ 1,855	$ 1,903	$ 1,749

Actual return on plan assets, separately identifying the amount related to assets sold during the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(02)(A)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)(iv)(02)(A)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e2446-114920

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- Definition

Actual return on plan assets, separately identifying the amount related to assets still held at the reporting date.

+ References

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=49178875&loc=d3e12355-114930

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

Amount of increase (decrease) in plan assets from purchases, sales and settlement of plan obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(02)(B)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)(iv)(02)(B)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e2446-114920

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- Definition

Transfers in and out of Level 3. For example, transfers due to changes in the observability of significant inputs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(02)(C)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
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-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e2446-114920

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