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Employee Benefit Plans - Narrative (Details) - USD ($) |  |  | 12 Months Ended
--- | --- | --- | ---
Jan. 01, 2020 | Jan. 01, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Employer contributions |  |  | $ 0 | $ 0 | $ 0
Expected contributions in the next fiscal year |  |  | $ 0 |  |
Percentage within which the planned allocation is managed |  |  | 5.00% |  |
Investments at fair value |  |  | $ 3,000,000,000 | 2,825,000,000 |
Growth-Oriented |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage of pension plan assets |  |  | 60.00% |  |
Investments at fair value |  |  | $ 1,816,000,000 | 2,027,000,000 |
Immunizing |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage of pension plan assets |  |  | 40.00% |  |
Investments at fair value |  |  | $ 1,184,000,000 | 798,000,000 |
Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Net actuarial losses (gains) |  |  | $ 197,000,000 | 418,000,000 |
Decrease in weighted average discount rate |  |  | 0.70% |  |
Investments at fair value |  |  | $ 3,000,000,000 | 2,825,000,000 | 2,431,000,000
Pre-tax (loss) gain to be amortized related to post-retirement obligations |  |  | (854,000,000) |  |
Expense associated with savings plans |  |  | 39,000,000 | 41,000,000 | 41,000,000
Post-retirement Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Life insurance and contributions to the medical program for eligible retirees will be eliminated (younger than) |  | 65 years |  |  |
Life insurance and contributions to the medical program for eligible retirees will be eliminated (65 years or older) | 65 years |  |  |  |
Decrease in post-retirement benefit liability as result of plan amendments |  |  |  |  | 102,000,000
Net actuarial losses (gains) |  |  | (11,000,000) | $ 22,000,000 |
Decrease in weighted average discount rate |  |  |  | 1.11% |
Investments at fair value |  |  | 0 | $ 0 | 0
Pre-tax (loss) gain to be amortized related to post-retirement obligations |  |  | 78,000,000 |  |
Premium expense incurred under the plan |  |  | $ 20,000,000 | $ 26,000,000 | 30,000,000
Post-retirement Benefits | Pension and Other Post-Employment Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Gain on decrease in post-retirement benefit liability due to plan amendments recognized in other comprehensive income |  |  |  |  | $ 102,000,000

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- Definition Target allocation percentage of investments in within which plan assets investment are managed. + References No definition available. + Details
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