Related Party Transactions - Schedule of related party transactions activity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013 management_company	Sep. 30, 2012 management_company	Sep. 30, 2013 management_company	Sep. 30, 2012 management_company
Related Party Transaction [Line Items]
Reported net premiums written	$ 1,402,062	$ 1,387,752	$ 5,043,584	$ 4,862,376
Net losses and loss expenses incurred	1,006,520	894,077	2,787,210	2,574,497
Reported acquisition costs	238,328	236,347	690,473	679,434
Number of investment management companies	5	4	5	4
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written	27,112	27,761	51,119	55,323
Net losses and loss expenses incurred	9,274	11,071	21,039	26,923
Reported acquisition costs	$ 7,189	$ 9,034	$ 17,875	$ 22,236