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Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Narrative) (Details) - USD ($) |  | 3 Months Ended | 12 Months Ended | 13 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Feb. 05, 2021 | Mar. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Feb. 05, 2021 | Dec. 31, 2011 | Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Pretax (favorable) unfavorable premium development, excluding Life & Group |  |  | $ 11,000,000 | $ (20,000,000) | $ (73,000,000) |  |  |
Funds held under reinsurance agreements, liability | $ 690,000,000 |  | 4,000,000,000 | 3,300,000,000 |  | $ 690,000,000 |  |
Retroactive reinsurance agreement, maximum | 1,000,000,000 |  |  |  |  | 1,000,000,000 |  |
Reinsurance premiums paid | $ 697,000,000 |  |  |  |  | $ 64,000,000 |  |
Reinsurance, loss on uncollectible accounts in period, amount |  | $ 12,000,000 |  |  |  |  |  |
Securities held as collateral, at fair value |  |  | 634,000,000 |  |  |  |  |
Corporate & Other |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Pretax (favorable) unfavorable premium development, excluding Life & Group |  |  | 60,000,000 | 50,000,000 | 38,000,000 |  |  |
Commercial |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Pretax (favorable) unfavorable premium development, excluding Life & Group |  |  | (6,000,000) | (7,000,000) | (40,000,000) |  |  |
Asbestos and Environmental Reserves |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO |  |  |  |  |  |  |  | $ 1,600,000,000
Aggregate limit under A&EP Loss Portfolio Transfer |  |  |  |  |  |  |  | 4,000,000,000
A&EP claim and allocated claim adjustment expense reserves ceded under existing third party reinsurance contracts transferred to NICO under A&EP Loss Portfolio Transfer |  |  |  |  |  |  |  | 1,200,000,000
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer |  |  |  |  |  |  |  | 2,000,000,000
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer |  |  |  |  |  |  |  | 215,000,000
Total consideration paid to NICO under AEP Loss Portfolio Transfer |  |  |  |  |  |  | $ 2,200,000,000 | $ 2,200,000,000
Net A&EP adverse development before consideration of LPT |  |  | 143,000,000 | 125,000,000 | 150,000,000 |  |  |
Provision for uncollectible third-party reinsurance, released |  |  | 5,000,000 | 25,000,000 | $ 25,000,000 |  |  |
Cumulative amounts ceded under AEP Loss Portfolio Transfer |  |  | 3,400,000,000 | 3,300,000,000 |  |  |  |
Deferred reinsurance benefit yet to be recognized |  |  | 429,000,000 | $ 398,000,000 |  |  |  |
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer |  |  | 3,100,000,000 |  |  |  |  |
Excess Workers' Compensation LPT |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Funds held under reinsurance agreements, liability |  |  | $ 690,000,000 |  |  |  |  |

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