Claim and Claim Adjustment Expense Reserves - A&EP Reserves (Details) - A&EP Reserves - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2010	Dec. 31, 2022
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	$ 15	$ 17	$ 38	$ 40
Cumulative amounts ceded under loss portfolio transfer	3,500		3,500			$ 3,500
Unrecognized deferred retroactive reinsurance benefit	388		388			$ 425
Fair value of collateral trust account	$ 2,300		$ 2,300