Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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	$ 30,762	$ 30,022	$ 28,210
Written premiums, assumed	1,001	1,041	1,042
Written premiums, ceded	(2,031)	(1,912)	(1,544)
Total net written premiums	29,732	29,151	27,708
Earned premiums, direct	29,978	28,994	27,536
Earned premiums, assumed	1,010	1,076	1,024
Earned premiums, ceded	(1,944)	(1,798)	(1,501)
Total net earned premiums	$ 29,044	$ 28,272	$ 27,059
Percentage of assumed earned premiums to net earned premiums	3.50%	3.80%	3.80%
Ceded claims and claim adjustment expenses incurred	$ 1,030	$ 1,089	$ 1,293
Gross reinsurance recoverables on paid and unpaid claims and claim adjustment expenses excluding structured settlements and mandatory pools and associations	3,731	3,476
Gross structured settlements	2,964	2,965
Mandatory pools and associations	1,801	1,886
Gross reinsurance recoverables	8,496	8,327
Allowance for uncollectible reinsurance	(146)	(92)
Net reinsurance recoverables	8,350	$ 8,235
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	$ 2,610
Terrorism Risk Insurance Program, annual cap limiting amount of aggregate subject losses for all participating insurers	$ 100,000