Investments in Affiliates - Narrative (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 2,177,645	$ 1,708,899
Additional investment commitment - other investments		59,300
Investment manager affiliates
Schedule of Equity Method Investments [Line Items]
Distributions received from investment manager affiliates		$ 35,900	25,300
Investment fund affiliates
Schedule of Equity Method Investments [Line Items]
Required notice period for redemption		30 days
Investments in affiliates		$ 1,674,925	$ 1,386,837
Ownership percentage remaining		5.50%	5.80%
Investment fund affiliates | Subject To Redemption Restrictions [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 553,200	$ 382,000
Investment fund affiliates | Funds held in side pockets
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		23,500	18,200
Investment fund affiliates | Hedge funds
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 1,317,375	$ 1,084,198
Ownership percentage remaining		4.80%	5.20%
Investment fund affiliates | Private investment funds
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 357,550	$ 302,639
Ownership percentage remaining		10.80%	9.50%
Operating affiliates
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 502,720	$ 322,062
Operating affiliates | CATGS I, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 17,500
Ownership percentage remaining		50.00%
Operating affiliates | Other strategic operating affiliates
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 446,695	251,348
Operating affiliates | Investment manager affiliates
Schedule of Equity Method Investments [Line Items]
Investments in affiliates	[1]	56,025	$ 70,714
Operating affiliates | Privilege Underwriters, Inc. [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 129,700
Operating affiliates | Privilege Underwriters' Reciprocal Exchange [Member]
Schedule of Equity Method Investments [Line Items]
Surplus Note Term		10 years
Additional investment commitment - other investments		$ 25,000
Operating affiliates | HighVista III, Ltd. [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 106,600
Ownership percentage remaining		29.50%
Operating affiliates | Westaim HIIG LP [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 33,100
Ownership percentage remaining		13.10%
Operating affiliates | Five Oaks Investment Corp [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 25,500
Ownership percentage remaining		23.10%
Cost of operating affiliate interests acquired		$ 49,200
Class of Warrant or Right, Number of Securities Called by Warrants or Rights		3,753
Class of Warrant or Right, Exercise Price of Warrants or Rights		$ 13.11
Operating affiliates | Common And Preferred Units [Member] | Privilege Underwriters, Inc. [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 77,500
Ownership percentage remaining		9.90%
Operating affiliates | Common And Preferred Units [Member] | Privilege Group Holdings GP, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage remaining		9.90%
Operating affiliates | Surplus Notes [Member] | Privilege Underwriters' Reciprocal Exchange [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 50,000
Surplus Note Term		10 years

[1] During the years ended December 31, 2016 and 2015, the Company received distributions from its Investment Manager Affiliates of approximately $35.9 million and $25.3 million, respectively.