Related Party Transactions - Schedule of related party transactions activity (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($) management_company	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) management_company	Jun. 30, 2014 USD ($)	Dec. 31, 2014 management_company
Related Party Transaction [Line Items]
Reported net premiums written		$ 2,118,331	$ 1,491,844	$ 3,969,580	$ 3,487,695
Net losses and loss expenses incurred		1,151,195	827,880	1,921,022	1,659,385
Reported acquisition costs	[1]	$ 341,617	184,619	$ 495,313	384,033
Number of investment management companies | management_company		6		6		5
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written		$ 4,445	18,025	$ 31,521	36,191
Net losses and loss expenses incurred		65	7,692	9,166	14,301
Reported acquisition costs		$ 1,384	$ 6,994	$ 10,395	$ 15,553

[1] The Company has reflected the amortization of certain fair value adjustments recorded in conjunction with the Catlin Acquisition within the respective segments.