Reinsurance (details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance disclosure
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	$ 26,648	$ 25,567	$ 24,939
Written premiums, assumed	1,000	928	843
Written premiums, ceded	(1,429)	(1,537)	(1,661)
Total net written premiums	26,219	24,958	24,121
Earned premiums, direct	26,189	25,262	24,740
Earned premiums, assumed	965	875	814
Earned premiums, ceded	(1,471)	(1,603)	(1,680)
Total net earned premiums	$ 25,683	$ 24,534	$ 23,874
Percentage of assumed earned premiums to net earned premiums	3.80%	3.60%	3.40%
Ceded claims and claim adjustment expenses incurred	$ 1,225	$ 762	$ 1,034
Gross reinsurance recoverables on paid and unpaid claims and claim adjustment expenses	3,303	3,181
Allowance for uncollectible reinsurance	(111)	(116)
Net reinsurance recoverables	3,192	3,065
Reinsurance recoverables, mandatory pools and associations	2,011	2,054
Reinsurance recoverables, structured settlements	3,106	3,168
Total reinsurance recoverables	8,309	$ 8,287
Terrorism Risk Insurance Program, annual aggregate industry loss minimum in order for a loss to be covered, next fiscal year	160
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	82.00%
Terrorism Risk Insurance Program, remaining life of the program (in years)	3 years
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	$ 2,460
Terrorism Risk Insurance Program, annual cap limiting amount of aggregate subject losses for all participating insurers	$ 100,000