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Pensions and Other Postretirement Benefits - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($) investment_firm | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan's accumulated benefit obligation | $ 2,600 | $ 2,300 |
Unfunded pension plans projected benefit obligation | 359 | 320 |
Unfunded pension plans accumulated benefit obligations | $ 330 | 292 |
Number of investment firms | investment_firm | 11 |  |
Multiemployer plan, contributions by employer | $ 22 | 31 | $ 35
Defined contribution plan, cost | 21 | 22 | $ 23
Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Estimated net loss for defined pension plans amortized from AOCI into net periodic cost | $ 66 |  |
Assumed increases in per capita cost of covered health care benefits | 6.00% |  |
Ultimate health care cost trend rate | 5.00% |  |
Pension plan assets target allocation for equity | 75.00% |  |
Assumed rate of return on pension plan assets | 8.00% |  |
Expected contribution for the next fiscal year | $ 19 |  |
OPEB |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, future amortization of prior service cost (credit) | 23 |  |
Expected contribution for the next fiscal year | 36 |  |
Fair Value Inputs Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Trust-owned life insurance fair value | $ 165 | $ 170 |
Debt securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Weighted average asset allocation, by asset category | 32.00% | 33.00% |
Defined Benefit Plan, Equity Securities | OPEB |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Weighted average asset allocation, by asset category | 68.00% | 67.00% |
Minimum |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets target allocation for equity | 50.00% |  |
Maximum |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets target allocation for equity | 75.00% |  |

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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + 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 (e) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + 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 (k)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920 + Details
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