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Fair Value Measurements - Schedule of Pension Plan Assets at Fair Value by Level (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | $ 3,000 | $ 2,825 |
Pension Plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 3,000 | 2,825 | $ 2,431
Pension Plans | Total |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 2,020 | 1,952 |
Pension Plans | Total | Common Stock |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 337 | 823 |
Pension Plans | Total | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 21 | 78 |
Pension Plans | Total | Cash and Cash Equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 387 | 229 |
Pension Plans | Total | Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 1,026 | 588 |
Pension Plans | Total | Government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 164 | 217 |
Pension Plans | Total | Asset-backed securities, derivatives and other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 85 | 17 |
Pension Plans | Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 745 | 1,130 |
Pension Plans | Level 1 | Common Stock |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 337 | 823 |
Pension Plans | Level 1 | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 21 | 78 |
Pension Plans | Level 1 | Cash and Cash Equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 387 | 229 |
Pension Plans | Level 1 | Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 0 | 0 |
Pension Plans | Level 1 | Government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 0 | 0 |
Pension Plans | Level 1 | Asset-backed securities, derivatives and other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 0 | 0 |
Pension Plans | Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 1,275 | 822 |
Pension Plans | Level 2 | Common Stock |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 0 | 0 |
Pension Plans | Level 2 | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 0 | 0 |
Pension Plans | Level 2 | Cash and Cash Equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 0 | 0 |
Pension Plans | Level 2 | Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 1,026 | 588 |
Pension Plans | Level 2 | Government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 164 | 217 |
Pension Plans | Level 2 | Asset-backed securities, derivatives and other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | 85 | 17 |
Pension Plans | Investments measured at net asset value |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investments at fair value | $ 980 | $ 873 |

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