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Employee Benefit Plans - Narrative (Details) |  |  | 12 Months Ended
--- | --- | --- | ---
Jan. 01, 2020 | Jan. 01, 2019 | Dec. 31, 2021 USD ($) plan | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Employer contributions |  |  | $ 0 | $ 0 | $ 0
Expected contributions expected in the next fiscal year |  |  | $ 0 |  |
Percentage within which the planned allocation is managed |  |  | 5.00% |  |
Investments at fair value |  |  | $ 3,016,000,000 | 3,000,000,000 |
Growth-Oriented |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage of pension plan assets |  |  | 60.00% |  |
Investments at fair value |  |  | $ 1,812,000,000 | 1,816,000,000 |
Immunizing |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target allocation percentage of pension plan assets |  |  | 40.00% |  |
Investments at fair value |  |  | $ 1,204,000,000 | 1,184,000,000 |
Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Net actuarial gain (loss) |  |  | $ 142,000,000 | (197,000,000) |
Increase (decrease) in weighted average discount rate |  |  | 0.35% |  |
Investments at fair value |  |  | $ 3,016,000,000 | 3,000,000,000 | 2,825,000,000
Pre-tax (loss) gain to be amortized related to post-retirement obligations |  |  | (637,000,000) |  |
Expense associated with savings plans |  |  | $ 29,000,000 | 39,000,000 | 41,000,000
Pension Plans | Quality Carriers, Inc. | Multiemployer Plan, Union-represented Employees |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Number of multi-employer pension plans | plan |  |  | 3 |  |
Contingent liability for full withdrawal or termination of multi-employer plan |  |  | $ 334,000,000 |  |
Pension Plans | Quality Carriers, Inc. | Multiemployer Plan, Union-represented Employees, Central Southeast And Southwest Areas Pension Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Contingent liability for full withdrawal or termination of multi-employer plan |  |  | 323,000,000 |  |
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 |  |  |  |
Net actuarial gain (loss) |  |  | 4,000,000 | $ 11,000,000 |
Increase (decrease) in weighted average discount rate |  |  |  | (0.70%) |
Investments at fair value |  |  | 0 | $ 0 | 0
Pre-tax (loss) gain to be amortized related to post-retirement obligations |  |  | 75,000,000 |  |
Premium expense incurred under the plan |  |  | $ 21,000,000 | $ 20,000,000 | $ 26,000,000

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