Losses and Loss Expense Reserve (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012 Sample	Dec. 31, 2011 Sample
Summary of balance of subrogation recoveries and the related claim liabilities, by estimation approach
Number of samples	45	46
Gross Claim Liabilities Before Subrogation Recoveries	$ 4,171,938	$ 3,777,581
Subrogation recoveries	(2,770,191)	(2,720,266)
Gross Claim Liabilities After Subrogation Recoveries	1,401,747	1,057,315
Adverse Samples [Member]
Summary of balance of subrogation recoveries and the related claim liabilities, by estimation approach
Number of samples	29	30
Gross Claim Liabilities Before Subrogation Recoveries	2,853,854	2,637,479
Subrogation recoveries	(1,553,195)	(1,457,472)
Gross Claim Liabilities After Subrogation Recoveries	1,300,659	1,180,006
Random Samples [Member]
Summary of balance of subrogation recoveries and the related claim liabilities, by estimation approach
Number of samples	16	16
Gross Claim Liabilities Before Subrogation Recoveries	1,318,084	1,140,102
Subrogation recoveries	(1,216,996)	(1,262,794)
Gross Claim Liabilities After Subrogation Recoveries	$ 101,088	$ (122,691)