Pension and Other Postretirement Employee Benefit Plans - Changes In Level 3 Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Defined benefit pension plans
Changes in level 3 fair value measurements using significant unobservable inputs
Fair value of plan assets, beginning of year	$ 19,603	$ 17,703
Fair value of plan assets, end of year	18,052	19,603
OPEB plans
Changes in level 3 fair value measurements using significant unobservable inputs
Fair value of plan assets, beginning of year	2,757	1,956
Fair value of plan assets, end of year	2,633	2,757
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	310	396
Actual return on plan assets, Realized gains/(losses)	0	0
Actual return on plan assets, Unrealized gains/(losses)	0	1
Purchases, sales and settlements, net	(1)	(87)
Transfers in and/or out of level 3	1	0
Fair value of plan assets, end of year	310	310
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	2,157	1,957
Actual return on plan assets, Realized gains/(losses)	0	0
Actual return on plan assets, Unrealized gains/(losses)	(85)	200
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	$ 2,072	$ 2,157

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

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- 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

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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-Publisher FASB
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
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-URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

+ Details

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- Definition

Amount, measured using unobservable input, of increase (decrease) in plan asset of defined benefit plan from purchase, sale and settlement of trade associated with underlying investment.

+ References

+ Details

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- 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

+ Details

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- Details

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- Details

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- Details

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- Details

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