Pension and Other Postretirement Employee Benefit Plans - Changes In Level 3 Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Defined benefit pension plans
Changes in level 3 fair value measurements using significant unobservable inputs
Fair value of plan assets, beginning of year	$ 17,703	$ 17,636
Fair value of plan assets, end of year	19,603	17,703
OPEB plans
Changes in level 3 fair value measurements using significant unobservable inputs
Fair value of plan assets, beginning of year	1,956	1,855
Fair value of plan assets, end of year	2,757	1,956
Level 3 \| Defined benefit pension plans \| U.S.
Changes in level 3 fair value measurements using significant unobservable inputs
Fair value of plan assets, beginning of year	396	539
Actual return on plan assets, Realized gains/(losses)	0	0
Actual return on plan assets, Unrealized gains/(losses)	1	(157)
Purchases, sales and settlements, net	(87)	0
Transfers in and/or out of level 3	0	14
Fair value of plan assets, end of year	310	396
Level 3 \| OPEB plans \| U.S.
Changes in level 3 fair value measurements using significant unobservable inputs
Fair value of plan assets, beginning of year	1,957	1,855
Actual return on plan assets, Realized gains/(losses)	0	0
Actual return on plan assets, Unrealized gains/(losses)	200	102
Purchases, sales and settlements, net	0	0
Transfers in and/or out of level 3	0	0
Fair value of plan assets, end of year	$ 2,157	$ 1,957

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold.

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)(5)(iv)(02)(A)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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Period Type:	duration

- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held.

+ References

+ Details

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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.

+ Details

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ 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 (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from purchases, sales and settlements.

+ 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)(5)(iv)(02)(B)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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Period Type:	duration

- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan, for assets transferred into (out of) Level 3 of fair value hierarchy. Includes, but is not limited to, transfer due to change in 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)(5)(iv)(02)(C)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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- Details

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- Details

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- Details

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