Related Party Transactions - Schedule of related party transactions activity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014 management_company	Jun. 30, 2013	Jun. 30, 2014 management_company	Jun. 30, 2013
Related Party Transaction [Line Items]
Reported net premiums written	$ 1,491,844	$ 1,533,868	$ 3,487,695	$ 3,641,522
Net losses and loss expenses incurred	827,880	937,606	1,659,385	1,780,690
Reported acquisition costs	184,619	231,886	384,033	452,145
Number of investment management companies	5		5
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written	18,025	11,415	36,191	24,155
Net losses and loss expenses incurred	7,692	6,117	14,301	11,766
Reported acquisition costs	$ 6,994	$ 5,355	$ 15,553	$ 10,707