Pension and Other Postretirement Employee Benefit Plans - Investment Strategy and Weighted Average Asset Allocation (Details) - USD ($) $ in Billions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Investments sponsored or managed by affiliates	$ 3.7	$ 6.0
Defined benefit pension plans
Asset class
Asset Allocation	100.00%
% of plan assets	100.00%	100.00%
Defined benefit pension plans \| Debt securities
Asset class
% of plan assets	48.00%	42.00%
Defined benefit pension plans \| Debt securities \| Minimum
Asset class
Asset Allocation	27.00%
Defined benefit pension plans \| Debt securities \| Maximum
Asset class
Asset Allocation	100.00%
Defined benefit pension plans \| Equity securities
Asset class
% of plan assets	37.00%	42.00%
Defined benefit pension plans \| Equity securities \| Minimum
Asset class
Asset Allocation	10.00%
Defined benefit pension plans \| Equity securities \| Maximum
Asset class
Asset Allocation	45.00%
Defined benefit pension plans \| Real estate
Asset class
% of plan assets	2.00%	3.00%
Defined benefit pension plans \| Real estate \| Minimum
Asset class
Asset Allocation	0.00%
Defined benefit pension plans \| Real estate \| Maximum
Asset class
Asset Allocation	10.00%
Defined benefit pension plans \| Alternatives
Asset class
% of plan assets	13.00%	13.00%
Defined benefit pension plans \| Alternatives \| Minimum
Asset class
Asset Allocation	0.00%
Defined benefit pension plans \| Alternatives \| Maximum
Asset class
Asset Allocation	35.00%
OPEB plans
Asset class
Asset Allocation		100.00%
% of plan assets	100.00%	100.00%
OPEB plans \| Debt securities
Asset class
% of plan assets	61.00%	61.00%
OPEB plans \| Debt securities \| Minimum
Asset class
Asset Allocation	30.00%
OPEB plans \| Debt securities \| Maximum
Asset class
Asset Allocation	70.00%
OPEB plans \| Equity securities
Asset class
% of plan assets	39.00%	39.00%
OPEB plans \| Equity securities \| Minimum
Asset class
Asset Allocation	30.00%
OPEB plans \| Equity securities \| Maximum
Asset class
Asset Allocation	70.00%
OPEB plans \| Real estate
Asset class
Asset Allocation	0.00%
% of plan assets	0.00%	0.00%
OPEB plans \| Alternatives
Asset class
Asset Allocation	0.00%
% of plan assets	0.00%	0.00%

Asset category.

No definition available.

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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=112275985&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=108410482&loc=d3e1928-114920

+ Details

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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=108410482&loc=d3e1928-114920

+ Details

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- Details

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