Claim and Claim Adjustment Expense Reserves - A&EP Reserves (Details) - A&EP Reserves - USD ($) $ in Millions |
3 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|
Mar. 31, 2025 |
Mar. 31, 2024 |
Dec. 31, 2010 |
Dec. 31, 2024 |
|
| A&EP Reserves [Abstract] | ||||
| Net A&EP claim and allocated claim adjustment expense reserves | $ 1,600 | |||
| Aggregate limit under A&EP loss portfolio transfer | 4,000 | |||
| Ceded A&EP claim and allocated claim adjustment expense reserves under existing third party reinsurance contracts transferred to NICO under A&EP loss portfolio transfer | 1,200 | |||
| Reinsurance premium paid to NICO under A&EP loss portfolio transfer | 2,000 | |||
| Net reinsurance receivables transferred to NICO under A&EP loss portfolio transfer | 215 | |||
| Total consideration | $ 2,200 | |||
| Retroactive reinsurance benefit recognized | $ 17 | $ 12 | ||
| Cumulative amounts ceded under loss portfolio transfer | 3,700 | $ 3,700 | ||
| Unrecognized deferred retroactive reinsurance benefit | 408 | $ 425 | ||
| Fair value of collateral trust account | $ 2,300 | |||