Reinsurance (Details) - Narrative (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reinsurance Disclosures [Abstract]
Reinsurance recoveries on losses and loss expenses recorded during period	$ 600,000,000	$ 1,100,000,000	$ 1,000,000,000
Reinsurance recoverable on paid losses	58,400,000	220,000,000
Unpaid losses and loss expenses recoverable	3,382,101,000	3,654,948,000
Total allowance for uncollectible reinsurance relating to bothreinsurance balances receivable and unpaid losses and loss expenses recoverable	107,900,000	99,200,000
Collateral held related to reinsurance recoverables	1,700,000,000
Total allowance recorded relating to reinsurance recoverables	53,300,000	55,300,000
Total allowance recorded relating to reinsurance balances receivable	54,600,000	43,900,000
Total reinsurance assets	$ 3,400,000,000	$ 3,900,000,000