Investments in Affiliates - Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 1,911,996	$ 2,177,645
Additional investment commitment - other investments		82,600
Investment manager affiliates
Schedule of Equity Method Investments [Line Items]
Distributions received from investment manager affiliates		$ 32,100	35,900
Investment fund affiliates
Schedule of Equity Method Investments [Line Items]
Required notice period for redemption		30 days
Investments in affiliates		$ 1,484,408	$ 1,674,925
Ownership percentage remaining		5.10%	5.50%
Investment fund affiliates | Subject To Redemption Restrictions [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 222,700	$ 553,200
Investment fund affiliates | Funds held in side pockets
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		20,100	23,500
Operating affiliates
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		427,588	502,720
Operating affiliates | Investment manager affiliates
Schedule of Equity Method Investments [Line Items]
Investments in affiliates	[1]	45,405	$ 56,025
Operating affiliates | Privilege Underwriters, Inc. [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 124,200
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 | Mahindra Insurance Brokers Limited [Member]
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 40,000
Ownership percentage remaining		20.00%
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, 2017 and 2016, the Company received distributions from its Investment Manager Affiliates of approximately $32.1 million and $35.9 million, respectively.