Claim and Claim Adjustment Expense Reserves - A&EP Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2010
Additional amounts ceded under LPT [Abstract]
Net A&EP adverse development before consideration of LPT	$ 86	$ 92	$ 143
Provision for uncollectible third-party reinsurance on A&EP		(5)	(5)
Total additional amounts ceded under LPT	86	87	138
Retroactive reinsurance benefit recognized	(94)	(91)	(107)
Pretax impact of deferred retroactive reinsurance	(8)	(4)	31
A&EP Reserves
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
Additional amounts ceded under LPT [Abstract]
Releases from provision for uncollectible third-party reinsurance	0	5	$ 5
Cumulative amounts ceded under loss portfolio transfer	3,600	3,500
Unrecognized deferred retroactive reinsurance benefit	417	$ 425
Fair value of collateral trust account	$ 2,500