Reinsurance (details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reinsurance disclosure
Written premiums, direct	$ 23,220,000,000	$ 22,634,000,000	$ 22,545,000,000
Written premiums, assumed	667,000,000	668,000,000	740,000,000
Written premiums, ceded	(1,700,000,000)	(1,667,000,000)	(1,949,000,000)
Total net written premiums	22,187,000,000	21,635,000,000	21,336,000,000
Earned premiums, direct	23,146,000,000	22,533,000,000	22,658,000,000
Earned premiums, assumed	641,000,000	664,000,000	762,000,000
Earned premiums, ceded	(1,697,000,000)	(1,765,000,000)	(2,002,000,000)
Total net earned premiums	22,090,000,000	21,432,000,000	21,418,000,000
Percentage of assumed earned premiums to net earned premiums	2.90%	3.10%	3.60%
Ceded claims and claim adjustment expenses incurred	737,000,000	404,000,000	582,000,000
Gross reinsurance recoverables on paid and unpaid claims and claim adjustment expenses	6,216,000,000	6,934,000,000
Allowance for uncollectible reinsurance	(345,000,000)	(363,000,000)
Net reinsurance recoverables	5,871,000,000	6,571,000,000
Reinsurance recoverables, mandatory pools and associations	2,020,000,000	2,043,000,000
Reinsurance recoverables, structured settlements	3,291,000,000	3,380,000,000
Total reinsurance recoverables	11,182,000,000	11,994,000,000
Terrorism Risk Insurance Program disclosure	The Terrorism Risk Insurance Program (the Program) is a Federal program administered by the Department of the Treasury that provides for a system of shared public and private compensation for certain insured losses resulting from acts of terrorism or war committed by or on behalf of a foreign interest. The Program has been authorized through 2014. In order for a loss to be covered under the Program (subject losses), the loss must meet certain aggregate industry loss minimums and must be the result of an event that is certified as an act of terrorism by the U.S. Secretary of the Treasury, in concurrence with the Secretary of State and the Attorney General of the United States. The annual aggregate industry loss minimum is $100 million through 2014. The Program excludes from participation the following types of insurance: Federal crop insurance, private mortgage insurance, financial guaranty insurance, medical malpractice insurance, health or life insurance, flood insurance, reinsurance, commercial automobile, professional liability (other than directors and officers'), surety, burglary and theft, and farm-owners multi-peril. In the case of a war declared by Congress, only workers' compensation losses are covered by the Program. All commercial property and casualty insurers licensed in the United States are generally required to participate in the Program. Under the Program, a participating insurer is entitled to be reimbursed by the Federal Government for 85% of subject losses, after an insurer deductible, subject to an annual cap. The deductible for any calendar year is equal to 20% of the insurer's direct earned premiums for covered lines for the preceding calendar year. The Company's estimated deductible under the Program is $2.17 billion for 2012. The annual cap limits the amount of aggregate subject losses for all participating insurers to $100 billion. Once subject losses have reached the $100 billion aggregate during a program year, participating insurers will not be liable under the Program for additional covered terrorism losses for that program year. The Company has had no terrorism-related losses since the Program was established. Since the interpretation of this law is untested, there is substantial uncertainty as to how they will be applied to specific circumstances. It is also possible that future legislative action could change the Program. Further, given the unpredictable frequency and severity of terrorism losses, as well as the limited terrorism coverage in the Company's own reinsurance program, future losses from acts of terrorism, particularly involving nuclear, biological, chemical or radiological events, could be material to the Company's operating results, financial position and/or liquidity in future periods. While the Company seeks to manage its exposure to man-made catastrophic events involving conventional means, there can be no assurance that the Company would have sufficient resources to respond to claims arising out of one or more man-made catastrophic events involving nuclear, biological, chemical or radiological means.
Terrorism Risk Insurance Program, Company's estimated deductible	2,170,000,000
Terrorism Risk Insurance Program, annual aggregate loss minimum in order for a loss to be covered	$ 100,000,000,000