Investments of Operating Entities and Consolidated Funds - Equity Method Investments - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Expense, equity method investments	$ 4,200	$ 7,100	$ 8,500
Equity method investments	40,171	41,099
Other than Temporary Impairment Losses, Investments	7,100
Surf House Ocean Views Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	7,589	14,179
Starboard Value LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	12,699	13,202
RCG Longview Debt Fund V Partners, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	11,000	9,284
RCG Longview Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,167	1,149
RCG Longview Debt Fund VI, LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,254	0
Healthcare Royalty GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	149	281
Healthcare Royalty GP II, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	176	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	1,573	764
Triartisan ES Partners, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,500	0
RCG Kennedy House, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	131	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	272	487
RCG Park Liberty GP Member Manager, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,248	428
Equity Method Investee, Other
Schedule of Equity Method Investments [Line Items]
Equity method investments	968	578
Equity Investment, Two
Schedule of Equity Method Investments [Line Items]
Assets	137,419	159,077
Liabilities	22,019	74,393
Equity	115,400	84,684
Revenue	79,870	136,459	87,707
Expenses	(35,835)	(32,425)	(34,490)
Net realized and unrealized gain (loss)	26,029	12,002	(6,305)
Net Income	70,064	116,036	46,912
Net Gains (Losses) on Securities, Derivatives and Other Investments
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments	$ 6,400	21,300	$ 14,400
Minimum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	15.00%
Maximum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	56.00%
Clawback Obligation
Schedule of Equity Method Investments [Line Items]
Contractual obligation		6,200
Clawback Obligation | RCG Longview Partners II, LLC
Schedule of Equity Method Investments [Line Items]
Contractual obligation	$ 6,500	$ 6,200