Pension and Other Postretirement Employee Benefit Plans - Investment Strategy and Weighted Average Asset Allocation (Details) - USD ($) $ in Billions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Investments sponsored or managed by affiliates	$ 2.7	$ 3.1
Defined benefit pension plans \| U.S.
Asset class
Asset Allocation	100.00%
% of plan assets	100.00%	100.00%
Defined benefit pension plans \| Debt securities \| U.S.
Asset class
% of plan assets	77.00%	74.00%
Defined benefit pension plans \| Debt securities \| Minimum \| U.S.
Asset class
Asset Allocation	42.00%
Defined benefit pension plans \| Debt securities \| Maximum \| U.S.
Asset class
Asset Allocation	100.00%
Defined benefit pension plans \| Equity securities \| U.S.
Asset class
% of plan assets	15.00%	16.00%
Defined benefit pension plans \| Equity securities \| Minimum \| U.S.
Asset class
Asset Allocation	0.00%
Defined benefit pension plans \| Equity securities \| Maximum \| U.S.
Asset class
Asset Allocation	40.00%
Defined benefit pension plans \| Real estate \| U.S.
Asset class
% of plan assets	1.00%	1.00%
Defined benefit pension plans \| Real estate \| Minimum \| U.S.
Asset class
Asset Allocation	0.00%
Defined benefit pension plans \| Real estate \| Maximum \| U.S.
Asset class
Asset Allocation	4.00%
Defined benefit pension plans \| Alternatives \| U.S.
Asset class
% of plan assets	7.00%	9.00%
Defined benefit pension plans \| Alternatives \| Minimum \| U.S.
Asset class
Asset Allocation	0.00%
Defined benefit pension plans \| Alternatives \| Maximum \| U.S.
Asset class
Asset Allocation	15.00%
OPEB plans \| Debt securities \| U.S.
Asset class
% of plan assets	59.00%	60.00%
OPEB plans \| Equity securities \| U.S.
Asset class
% of plan assets	41.00%	40.00%

Asset category.

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- Definition

For defined benefit and OPEB plans, amount of plan investments held in funds that are sponsored or managed by affiliates of the entity.

+ References

No definition available.

+ 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

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

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

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