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Pension and Other Postretirement Benefit Plans - Fair Values of Other Postretirement Benefit Plan Assets (Details) - Other Postretirement Benefits - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 1,102	$ 968	$ 1,114
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	74	94
Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	343	295
Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	81	79
Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	17	16
Corporate obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	3
Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	225	200
Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	196	141
Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	149	116
Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	17	24
Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	312	307
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	52	78
Level 1 | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	19	16
Level 1 | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	15	12
Level 1 | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1	1
Level 1 | Corporate obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 1 | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	225	200
Level 1 | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 1 | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 1 | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	362	281
Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2 | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2 | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2 | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2 | Corporate obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2 | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2 | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	196	141
Level 2 | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	149	116
Level 2 | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	17	24
Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 3 | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 3 | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 3 | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 3 | Corporate obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 3 | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 3 | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 3 | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 3 | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
NAV
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	428	380
NAV | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	22	16
NAV | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	324	279
NAV | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	66	67
NAV | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	16	15
NAV | Corporate obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	3
NAV | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
NAV | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
NAV | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
NAV | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 0	$ 0

X

- Definition

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+ References

No definition available.

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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
-Subparagraph (d)(iv)(01)
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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-Paragraph 1
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