Related Party Transactions - Schedule of related party transactions activity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013 management_company	Jun. 30, 2012 management_company	Jun. 30, 2013 management_company	Jun. 30, 2012 management_company
Related Party Transaction [Line Items]
Reported net premiums written	$ 1,533,868	$ 1,432,900	$ 3,641,522	$ 3,474,624
Net losses and loss expenses incurred	937,606	826,354	1,780,690	1,680,420
Reported acquisition costs	231,886	218,937	452,145	443,087
Number of investment management companies	5	4	5	4
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written	11,420	10,972	24,007	27,561
Net losses and loss expenses incurred	6,117	7,728	11,766	15,851
Reported acquisition costs	$ 5,355	$ 6,415	$ 10,685	$ 13,202