Related Party Transactions - Schedule of related party transactions activity (Details) $ in Thousands			3 Months Ended		6 Months Ended
		Jul. 24, 2015 USD ($)	Jun. 30, 2016 USD ($) management_company	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) management_company	Jun. 30, 2015 USD ($)	Dec. 31, 2015 management_company
Related Party Transaction [Line Items]
Reported net premiums written			$ 2,726,119	$ 2,118,331	$ 5,790,878	$ 3,969,580
Net losses and loss expenses incurred			1,632,386	1,151,195	3,014,871	1,921,022
Reported acquisition costs	[1]		$ 420,520	341,617	$ 823,787	495,313
Number of investment management companies | management_company			4		4		6
Related Party Transactions Number of Investment Technology Service Firms | management_company			1		1		2
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written			$ 21,889	4,445	$ 45,905	31,521
Net losses and loss expenses incurred			3,836	65	15,896	9,166
Reported acquisition costs			$ 7,831	$ 1,384	$ 19,435	$ 10,395
New Energy Risk Inc.
Related Party Transaction [Line Items]
Percentage of voting interests acquired		63.63%
Ownership percentage held by noncontrolling interest		5.21%
Cash consideration		$ 8,800
Ownership percentage following additional purchase of shares		94.79%

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