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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, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | $ 1,221 | $ 1,102 | $ 968
Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 59 | 74 |
Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 374 | 343 |
Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 101 | 81 |
Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 1 | 17 |
Fixed income obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 8 | 0 |
Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 258 | 225 |
Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 221 | 196 |
Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 196 | 149 |
Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 3 | 17 |
Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 325 | 312 |
Level 1 | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 31 | 52 |
Level 1 | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 19 | 19 |
Level 1 | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 16 | 15 |
Level 1 | Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 1 | 1 |
Level 1 | Fixed income 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 | 258 | 225 |
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 | 428 | 362 |
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 | Fixed income obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 8 | 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 | 221 | 196 |
Level 2 | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 196 | 149 |
Level 2 | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 3 | 17 |
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 | Fixed income 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 | 468 | 428 |
NAV | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 28 | 22 |
NAV | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 355 | 324 |
NAV | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 85 | 66 |
NAV | Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 16 |
NAV | Fixed income obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
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 |

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