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Pensions and Other Postretirement Benefits - Fair Value of Plan Assets (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | $ 2,675 | $ 2,462
Fair Value Inputs Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 1,543 | 1,379
Fair Value Inputs Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 1,132 | 1,083
Common Stock |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 1,483 | 1,329
Common Stock | Fair Value Inputs Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 1,483 | 1,329
Common Stock | Fair Value Inputs Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 0 | 0
International equity securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 399 | 377
International equity securities | Fair Value Inputs Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 0 | 0
International equity securities | Fair Value Inputs Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 399 | 377
Debt securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 297 | 303
Debt securities | Fair Value Inputs Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 0 | 0
Debt securities | Fair Value Inputs Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 297 | 303
U.S. government and agencies securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 146 | 172
U.S. government and agencies securities | Fair Value Inputs Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 0 | 0
U.S. government and agencies securities | Fair Value Inputs Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 146 | 172
Corporate Bonds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 117 | 84
Corporate Bonds | Fair Value Inputs Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 0 | 0
Corporate Bonds | Fair Value Inputs Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 117 | 84
Mortgage and Other Asset-backed Securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 24 | 26
Mortgage and Other Asset-backed Securities | Fair Value Inputs Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 0 | 0
Mortgage and Other Asset-backed Securities | Fair Value Inputs Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 24 | 26
Commingled Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 149 | 121
Commingled Funds | Fair Value Inputs Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 0 | 0
Commingled Funds | Fair Value Inputs Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 149 | 121
Cash and cash equivalents |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 60 | 50
Cash and cash equivalents | Fair Value Inputs Level 1 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | 60 | 50
Cash and cash equivalents | Fair Value Inputs Level 2 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets | $ 0 | $ 0

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