Reinsurance Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	$ 28,210	$ 26,648	$ 25,567
Written premiums, assumed	1,042	1,000	928
Written premiums, ceded	(1,544)	(1,429)	(1,537)
Total net written premiums	27,708	26,219	24,958
Earned premiums, direct	27,536	26,189	25,262
Earned premiums, assumed	1,024	965	875
Earned premiums, ceded	(1,501)	(1,471)	(1,603)
Total net earned premiums	$ 27,059	$ 25,683	$ 24,534
Percentage of assumed earned premiums to net earned premiums	3.80%	3.80%	3.60%
Ceded claims and claim adjustment expenses incurred	$ 1,293	$ 1,225	$ 762
Gross reinsurance recoverables on paid and unpaid claims and claim adjustment expenses	3,485	3,303
Allowance for uncollectible reinsurance	(110)	(111)
Net reinsurance recoverables	3,375	3,192
Mandatory pools and associations	2,005	2,011
Structured settlements	2,990	3,106
Total reinsurance recoverables	8,370	$ 8,309
Terrorism Risk Insurance Program, annual aggregate industry loss minimum in order for a loss to be covered, next fiscal year	180
Terrorism Risk Insurance Program, ultimate 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, next fiscal year	81.00%
Terrorism Risk Insurance Program, ultimate 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	$ 2,520
Terrorism Risk Insurance Program, annual cap limiting amount of aggregate subject losses for all participating insurers	$ 100,000