Employee Pension and Other Postretirement Benefits - Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Change in fair value of plan assets
Fair value of plan assets, beginning of year	$ 3,854	$ 3,254
Actual return on plan assets	(196)	608
Employer contributions	0	115
Plan participants' contributions	0	0
Benefits paid	(129)	(123)
Fair value of plan assets, end of year	$ 3,529	$ 3,854
Pension Benefits \| Equity securities
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation	53.00%	63.00%
Pension Benefits \| Debt Securities
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation	35.00%	25.00%
Pension Benefits \| Real estate
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation	12.00%	12.00%
Other Benefits
Change in fair value of plan assets
Fair value of plan assets, beginning of year	$ 288	$ 260
Actual return on plan assets	(18)	44
Employer contributions	0	1
Plan participants' contributions	4	4
Benefits paid	(19)	(21)
Fair value of plan assets, end of year	$ 255	$ 288
Other Benefits \| Equity securities
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation	70.00%
Other Benefits \| Debt Securities
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation	30.00%

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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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

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 50
-Paragraph 1
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ 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

+ Details

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- Definition

Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(5)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount of contributions received by defined benefit plan from participant which increases plan assets.

+ References

+ Details

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- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

+ Details

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- Details

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