Investments in Affiliates - Investment Fund Affiliates (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		$ 1,911,996	$ 2,177,645
Equity in net income (loss) in operating affiliates		202,691	115,155	$ 118,060
Investment fund affiliates
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		1,484,408	1,674,925
Equity in net income (loss) in operating affiliates		$ 139,046	$ 70,758
Weighted average percentage ownership		5.10%	5.50%
Combined funds/ estimated total net assets	[1]	$ 31,442,671	$ 30,600,053
Investment fund affiliates | Hedge funds
Schedule of Equity Method Investments [Line Items]
Investments in affiliates	[2]	1,119,747	1,317,375
Equity in net income (loss) in operating affiliates	[2]	$ 104,169	$ 41,670
Weighted average percentage ownership	[2]	4.00%	4.80%
Combined funds/ estimated total net assets	[1],[2]	$ 27,785,552	$ 27,281,980
Investment fund affiliates | Private investments
Schedule of Equity Method Investments [Line Items]
Investments in affiliates		364,661	357,550
Equity in net income (loss) in operating affiliates		$ 34,877	$ 29,088
Weighted average percentage ownership		10.20%	10.80%
Combined funds/ estimated total net assets	[1]	$ 3,657,119	$ 3,318,073

[1] Total estimated net assets are generally as at November 30 for hedge fund affiliates and September 30 for private investment affiliates.

[2] The Company accounts for its investment in certain funds using the equity method where the Company has significant influence over the related investment management company.