Claim and Claim Adjustment Expense Reserves - A&EP Reserves (Details) - A&EP Reserves - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2010	Dec. 31, 2023
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	$ 12	$ 8
Cumulative amounts ceded under loss portfolio transfer	3,600			$ 3,600
Unrecognized deferred retroactive reinsurance benefit	405			$ 417
Fair value of collateral trust account	$ 2,600