Investments - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2010
Schedule of Investments [Line Items]
Income from Equity Method Investments	$ 2,211,000	$ 2,125,000
Equity Method Investment	58,819,000		$ 58,507,000
Marketable Securities	213,483,000		204,627,000
Net Realized and Unrealized Gains (Losses) on Private Equity Fund Investments, Including Performance Fees	(4,000)	346,000
Previously Received Carried Interest Subject to Repayment	0
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	4,959,000		5,445,000
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	7,540,000		8,048,000
Unfunded Commitments for Capital Contributions	12,910,000
Equity Securities without Readily Determinable Fair Value, Amount	1,079,000		1,079,000
Equity Method Investments [Member]
Schedule of Investments [Line Items]
Amortization of Intangible Assets	171,000	222,000
Private Equity Funds [Member]
Schedule of Investments [Line Items]
Equity Method Investment	$ 14,382,000		9,800,000
ABS [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	46.00%
Income from Equity Method Investments	$ 1,731,000	1,872,000
Equity Method Investment	$ 39,787,000		38,699,000
Atalanta Sosnoff [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	49.00%
Income from Equity Method Investments	$ 225,000	$ 253,000
Equity Method Investment	$ 12,858,000		13,291,000
Luminis [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%
Income from Equity Method Investments	$ 255,000
Equity Method Investment	6,174,000		6,517,000
G5 [Member]
Schedule of Investments [Line Items]
Debt Securities, Held-to-maturity	9,227,000		9,717,000
Trilantic VI [Member]
Schedule of Investments [Line Items]
Other Ownership Interests, Contributed Capital	2,313,000
Investment	2,313,000
Capital Commitment	12,000,000
Unfunded Commitments for Capital Contributions	$ 9,687,000
ECP II [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Minimum [Member]
Schedule of Investments [Line Items]
Percent Ownership of Carried Interest	8.00%
ECP II [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Maximum [Member]
Schedule of Investments [Line Items]
Percent Ownership of Carried Interest	9.00%
ECP II [Member] | Private Equity Funds [Member]
Schedule of Investments [Line Items]
Equity Method Investment	$ 774,000		795,000
Cash	770,000
Marketable Securities	4,000
Trilantic [Member]
Schedule of Investments [Line Items]
Issued LP Units (in shares)				500,000
Limited Partnership Investment				$ 16,090,000
Investment	7,546,000		9,932,000
Trilantic IV [Member]
Schedule of Investments [Line Items]
Investment			1,178,000
Trilantic V [Member]
Schedule of Investments [Line Items]
Investment	72,000		4,980,000
Capital Commitment	5,000,000
Unfunded Commitments for Capital Contributions	582,000
Glisco [Member]
Schedule of Investments [Line Items]
Equity Securities without Readily Determinable Fair Value, Amount	$ 1,253,000		$ 1,609,000