Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
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	$ 40,983	$ 36,648	$ 33,180
Written premiums, assumed	1,989	1,228	1,064
Written premiums, ceded	(2,771)	(2,462)	(2,285)
Total net written premiums	40,201	35,414	31,959
Earned premiums, direct	38,796	34,948	31,977
Earned premiums, assumed	1,614	1,145	1,032
Earned premiums, ceded	(2,649)	(2,330)	(2,154)
Total net earned premiums	$ 37,761	$ 33,763	$ 30,855
Percentage of assumed earned premiums to net earned premiums	4.30%	3.40%	3.30%
Ceded claims and claim adjustment expenses incurred	$ 1,462	$ 1,187	$ 1,184
Gross reinsurance recoverables on paid and unpaid claims and claim adjustment expenses excluding structured settlements and mandatory pools and associations	3,895	3,792
Gross structured settlements	2,707	2,802
Mandatory pools and associations	1,659	1,601
Gross reinsurance recoverables	8,261	8,195
Allowance for estimated uncollectible reinsurance	(118)	(132)	(141)
Net reinsurance recoverables	8,143	$ 8,063	$ 8,452
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	$ 3,480
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