Investments of Operating Entities and Consolidated Funds - Equity Method Investments - (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Expense, equity method investments	$ 1,900	$ 1,100
Equity method investments	$ 45,171		$ 41,099
Number of investments	0
Net Gains (Losses) on Securities, Derivatives and Other Investments
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments	$ 3,900	$ 3,100
Surf House Ocean Views Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	14,179		14,179
Starboard Value LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	16,274		13,202
RCG Longview Debt Fund V Partners, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	9,306		9,284
RCG Longview Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,212		1,149
Healthcare Royalty GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	249		281
Healthcare Royalty GP II, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	142		148
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	1,108		764
RCG Kennedy House, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	119		154
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	270		487
RCG Park Liberty GP Member Manager, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,018		428
Equity Method Investee, Other
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 849		578
Equity Method Investee, Exceeded Threshold for Income Test [Member]
Schedule of Equity Method Investments [Line Items]
Number of investments	1
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