Investments of Operating Entities and Consolidated Funds - Equity Method Investments - (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Equity Method Investments [Line Items]
Expense, equity method investments	$ 7,100	$ 8,500	$ 14,100
Equity method investments	$ 41,099	36,991
Number of investments	3
Net Gains (Losses) on Securities, Derivatives and Other Investments
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments	$ 21,300	14,400	3,400
Surf House Ocean Views Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	14,179	13,522
Starboard Value LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	13,202	12,501
RCG Longview Debt Fund V Partners, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	9,284	7,256
RCG Longview Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,149	656
Healthcare Royalty GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	281	583
Healthcare Royalty GP II, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	148	354
RCG Longview Debt Fund IV Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	331	331
Healthcare Royalty GP III, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	764	208
RCG Kennedy House, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	154	183
RCG Longview Equity Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	114	114
RCG LPP II GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	487	0
RCG Park Liberty GP Member Manager, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	428	141
Equity Method Investee, Other
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 578	1,142
Equity Method Investee, Exceeded Threshold for Income Test [Member]
Schedule of Equity Method Investments [Line Items]
Number of investments	1
Cash	$ 6,046	5,766
Performance & management fee receivable	110,064	48,145
Investments in Portfolio Funds, at fair value	11,251	7,493
Other assets	1,060	1,122
Liabilities	74,256	21,250
Equity	54,165	41,276
Revenues	132,862	87,371	35,268
Expenses	(32,354)	(34,490)	(20,658)
Net realized and unrealized gains (losses)	9,264	9,383	4,259
Net Income	109,772	62,264	18,869
Equity Investment, Two
Schedule of Equity Method Investments [Line Items]
Assets	155,936	88,965
Liabilities	74,316	22,504
Equity	81,620	66,461
Revenues	137,052	90,337	38,571
Expenses	(32,354)	(34,490)	(20,658)
Net realized and unrealized gains (losses)	10,149	(6,305)	9,715
Net Income	$ 114,847	49,542	$ 27,628
Minimum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%
Maximum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	57.00%
Clawback Obligation [Member]
Schedule of Equity Method Investments [Line Items]
Contractual obligation		6,200
Clawback Obligation [Member] | RCG Longview Partners II, LLC
Schedule of Equity Method Investments [Line Items]
Contractual obligation	$ 6,200	$ 6,200