Investments - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2010
Schedule of Investments [Line Items]
Income from Equity Method Investments	$ 2,125,000	$ 1,610,000
Equity Method Investment	60,241,000		$ 59,856,000
Marketable Securities	96,350,000		65,032,000
Net Realized and Unrealized Gains (Losses) on Private Equity Fund Investments, Including Performance Fees	346,000	339,000
Previously Received Carried Interest Subject to Repayment	0
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	7,950,000		8,730,000
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	10,153,000		10,996,000
Unfunded Commitments for Capital Contributions	3,206,000
Equity Securities without Readily Determinable Fair Value, Amount	$ 1,079,000		1,079,000
ABS [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	45.00%
Income from Equity Method Investments	$ 1,872,000	1,531,000
Equity Method Investment	$ 40,440,000		39,894,000
Atalanta Sosnoff [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	49.00%
Income from Equity Method Investments	$ 253,000	105,000
Equity Method Investment	$ 13,802,000		13,963,000
Luminis [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	19.00%
Equity Method Investment	$ 5,999,000		5,999,000
Equity Method Investments [Member]
Schedule of Investments [Line Items]
Amortization of Intangible Assets	$ 222,000	$ 390,000
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%
G5 [Member]
Schedule of Investments [Line Items]
Held-to-maturity Securities	$ 11,132,000		10,995,000
Trilantic [Member]
Schedule of Investments [Line Items]
Issued LP Units (in shares)				500,000
Limited Partnership Investment				$ 16,090,000
Investment	10,092,000		10,399,000
Participation in Successor Funds	12,000,000
Trilantic IV [Member]
Schedule of Investments [Line Items]
Investment			1,178,000
Trilantic V [Member]
Schedule of Investments [Line Items]
Investment	307,000		4,513,000
Capital Commitment	5,000,000
Unfunded Commitments for Capital Contributions	782,000
Glisco [Member]
Schedule of Investments [Line Items]
Equity Securities without Readily Determinable Fair Value, Amount	2,037,000		2,172,000
Private Equity Funds [Member]
Schedule of Investments [Line Items]
Equity Method Investment	13,561,000		13,812,000
Private Equity Funds [Member] | ECP II [Member]
Schedule of Investments [Line Items]
Equity Method Investment	809,000		$ 833,000
Cash	782,000
Marketable Securities	$ 27,000