Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Reinsurance Disclosures [Abstract]
Percentage of residual market business written directly by the Company for mandatory pools and associations and then ceded to the mandatory pool	100.00%
Written premiums, direct	$ 44,377	$ 40,983	$ 36,648
Written premiums, assumed	2,173	1,989	1,228
Written premiums, ceded	(3,194)	(2,771)	(2,462)
Total net written premiums	43,356	40,201	35,414
Earned premiums, direct	42,983	38,796	34,948
Earned premiums, assumed	2,095	1,614	1,145
Earned premiums, ceded	(3,137)	(2,649)	(2,330)
Total net earned premiums	$ 41,941	$ 37,761	$ 33,763
Percentage of assumed earned premiums to net earned premiums	5.00%	4.30%	3.40%
Ceded claims and claim adjustment expenses incurred	$ 1,249	$ 1,462	$ 1,187
Gross reinsurance recoverables on paid and unpaid claims and claim adjustment expenses excluding structured settlements and mandatory pools and associations	3,962	3,895
Gross structured settlements	2,626	2,707
Mandatory pools and associations	1,531	1,659
Gross reinsurance recoverables	8,119	8,261
Allowance for estimated uncollectible reinsurance	(119)	(118)	(132)
Net reinsurance recoverables	8,000	$ 8,143	$ 8,063
Terrorism Risk Insurance Program, annual aggregate industry loss minimum in order for a loss to be covered	$ 200
Terrorism Risk Insurance Program, percentage of subject losses reimbursed by the Federal Government, after insurer deductible, subject to annual cap	80.00%
Terrorism Risk Insurance Program, percentage of insurer's direct earned premiums for covered lines for the preceding calendar year at which the deductible is set	20.00%
Terrorism Risk Insurance Program, Company's estimated deductible for the next fiscal year	$ 3,850
Terrorism Risk Insurance Program, annual cap limiting amount of aggregate subject losses for all participating insurers	100,000
Terrorism Risk Insurance Program, terrorism-related losses triggering program coverage since program was established	$ 0