Investments - Additional Information (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2010
Schedule of Investments [Line Items]
Income from Equity Method Investments	$ 5,846,000	$ 3,111,000	$ 2,313,000	$ 3,128,000	$ 3,770,000	$ 2,562,000	$ 2,453,000	$ 2,211,000	$ 14,398,000	$ 10,996,000	$ 9,294,000
Amortization of Intangible Assets									1,605,000	8,077,000	9,199,000
Net Realized and Unrealized Gains (Losses) on Private Equity Fund Investments									(1,388,000)	(790,000)	(397,000)
Previously Received Carried Interest Subject to Repayment	368,000								368,000
Total Assets	3,370,888,000				2,598,613,000				3,370,888,000	2,598,613,000	2,125,667,000
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	5,572,000				8,810,000				5,572,000	8,810,000
Unfunded Commitments for Capital Contributions	11,975,000								11,975,000
Equity Securities without Readily Determinable Fair Value, Amount	683,000				1,772,000				683,000	1,772,000
Variable Interest Entity, Not Primary Beneficiary [Member]
Schedule of Investments [Line Items]
Total Assets	$ 3,083,000				4,658,000				3,083,000	4,658,000
Equity Method Investments [Member]
Schedule of Investments [Line Items]
Amortization of Intangible Assets									$ 316,000	684,000	893,000
ABS [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	46.00%								46.00%
Income from Equity Method Investments									$ 10,855,000	8,870,000	7,565,000
Atalanta Sosnoff [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	49.00%								49.00%
Income from Equity Method Investments									$ 1,997,000	1,210,000	1,211,000
Luminis [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%								20.00%
Income from Equity Method Investments									$ 1,546,000	916,000	$ 518,000
G5 [Member]
Schedule of Investments [Line Items]
Debt Securities, Held-to-maturity	$ 7,385,000				9,235,000				7,385,000	9,235,000
Trilantic [Member]
Schedule of Investments [Line Items]
Issued LP Units (in shares)												500,000
Limited Partnership Investment												$ 16,090,000
Investment	6,623,000				6,762,000				6,623,000	6,762,000
Trilantic IV [Member]
Schedule of Investments [Line Items]
Investment					1,178,000					1,178,000
Trilantic V [Member]
Schedule of Investments [Line Items]
Investment	29,000				5,135,000				29,000	5,135,000
Capital Commitment	5,000,000								5,000,000
Unfunded Commitments for Capital Contributions	372,000								372,000
Trilantic VI [Member]
Schedule of Investments [Line Items]
Investment	110,000				3,015,000				110,000	3,015,000
Capital Commitment	12,000,000								12,000,000
Unfunded Commitments for Capital Contributions	9,054,000								9,054,000
Glisco [Member]
Schedule of Investments [Line Items]
Equity Securities without Readily Determinable Fair Value, Amount	$ 387,000				$ 899,000				$ 387,000	$ 899,000