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Acquisition and Disposition - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions |  |  |  |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 15, 2021 | Feb. 03, 2021 | Jan. 26, 2021 | Jan. 04, 2021 | Mar. 31, 2021 | Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2021 | Mar. 29, 2021 | Dec. 31, 2020
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  | $ 3,350 | $ 3,350 |  |  |  | $ 2,369
Long-term debt |  |  |  |  | 7,996 | 7,996 |  |  |  | 7,825
Amortization of intangible assets |  |  |  |  |  | 53 | $ 28 |  |  |
Redemption of preferred stock |  |  |  |  | $ 250 |  |  |  |  |
Proceeds from affiliates |  |  | $ 2,800 |  |  |  |  |  |  |
Definitive agreement with Wilton Reassurance |  |  |  |  |  |  |  |  | $ 220 |
Loss on disposition of operations |  |  |  |  |  | $ 3,998 | $ 0 |  |  |
Noncumulative Preferred Stock | Series A |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Preferred stock, redemption price par value (in dollars per share) |  |  |  |  | $ 0.01 | $ 0.01 |  |  |  |
Preferred stock, dividend rate, percentage |  |  |  |  | 7.50% |  |  |  |  |
Noncumulative Preferred Stock | Series B |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Preferred stock, redemption price par value (in dollars per share) |  |  |  |  | $ 0.01 | 0.01 |  |  |  |
Preferred stock, dividend rate, percentage |  |  |  |  | 7.50% |  |  |  |  |
Non-Cumulative Convertible Preferred Stock | Series D |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Preferred stock, redemption price par value (in dollars per share) |  |  |  |  | $ 0.01 | $ 0.01 |  |  |  |
Discontinued Operations, Held-for-sale |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Amortized cost, net |  |  |  |  | $ 23,511 | $ 23,511 |  |  |  | 21,417
Equity securities, at fair value, cost |  |  |  |  | 1,191 | 1,191 |  |  |  | 1,113
Other short term Investments, amortized cost |  |  |  |  | 1,652 | 1,652 |  |  |  | $ 993
Subordinated Debt |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Repaid of subordinated notes | $ 72 | $ 100 |  |  |  |  |  |  |  |
Subsequent acquisition (in percentage) |  | 7.625% |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  | $ 417 | $ 417 |  |  |  |
Senior Notes |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Subsequent acquisition (in percentage) |  |  |  |  | 6.75% | 6.75% |  |  |  |
National General Holdings Corp. |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  | $ 980 |  |  |  |  |  |
Purchase accounting fair value adjustment |  |  |  |  |  | $ 67 |  |  |  |
Amortization of intangible assets |  |  |  |  |  | 23 |  |  |  |
Amortization expense for the remainder of 2021 |  |  |  |  | $ 228 | 228 |  |  |  |
Amortization expense of present value of future profits |  |  |  |  |  | 134 |  |  |  |
Fair value purchase accounting adjustments, increase in reserves |  |  |  |  |  | 74 |  |  |  |
Fair value purchase accounting adjustments, reduction in investments |  |  |  |  |  | 13 |  |  |  |
Fair value purchase accounting adjustments, increase in current and deferred tax liabilities |  |  |  |  |  | 98 |  |  |  |
Business acquisition, transaction costs |  |  |  |  | 22 | $ 22 |  |  |  |
National General Holdings Corp. | Forecast |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Amortization expense of present value of future profits |  |  |  |  |  |  |  | $ 183 |  |
National General Holdings Corp. | Minimum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Finite-lived intangible asset, useful life (in years) |  |  |  | 3 years |  | 6 months |  |  |  |
National General Holdings Corp. | Maximum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Finite-lived intangible asset, useful life (in years) |  |  |  | 10 years |  | 12 months |  |  |  |
National General Holdings Corp. | Allstate Protection |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  | $ 616 |  |  |  |  |  |
Present value of future profits |  |  |  |  | $ 317 | $ 317 |  |  |  |
National General Holdings Corp. | Protection Services |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  | 29 |  |  |  |  |  |
National General Holdings Corp. | Allstate Health and Benefits |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  | $ 335 |  |  |  |  |  |

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