Related Party Transactions - Schedule of related party transactions activity (Details) $ in Thousands		3 Months Ended			9 Months Ended
	Jul. 24, 2015 USD ($)	Sep. 30, 2015 USD ($) management_company		Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) management_company		Sep. 30, 2014 USD ($)	Dec. 31, 2014 management_company
Related Party Transaction [Line Items]
Reported net premiums written		$ 2,091,028		$ 1,233,985	$ 6,060,608		$ 4,721,680
Net losses and loss expenses incurred		1,464,285		859,588	3,385,307		2,518,973
Reported acquisition costs		$ 409,173	[1]	182,882	$ 904,486	[1]	566,915
Number of investment management companies | management_company		6			6			6
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written		$ 5,075		31,981	$ 36,596		68,172
Net losses and loss expenses incurred		1,402		12,738	10,567		27,038
Reported acquisition costs		$ 1,874		$ 10,969	$ 12,269		$ 26,522
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.