Investments - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2010
Schedule of Investments [Line Items]
Income from Equity Method Investments	$ 2,419,000	$ 2,070,000	$ 4,544,000	$ 3,680,000
Equity Method Investment	56,184,000		56,184,000		$ 59,856,000
Marketable Securities	199,929,000		199,929,000		65,032,000
Net Realized and Unrealized Gains (Losses) on Private Equity Fund Investments, Including Performance Fees	(204,000)	(2,532,000)	142,000	(2,193,000)
Previously Received Carried Interest Subject to Repayment	0		0
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	6,208,000		6,208,000		8,730,000
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	8,721,000		8,721,000		10,996,000
Unfunded Commitments for Capital Contributions	3,255,000		3,255,000
Equity Securities without Readily Determinable Fair Value, Amount	1,079,000		1,079,000		1,079,000
Private Equity Funds [Member]
Schedule of Investments [Line Items]
Equity Method Investment	$ 10,663,000		$ 10,663,000		13,812,000
ABS [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	46.00%		46.00%
Income from Equity Method Investments	$ 1,891,000	1,712,000	$ 3,763,000	3,243,000
Equity Method Investment	$ 36,326,000		$ 36,326,000		39,894,000
Atalanta Sosnoff [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	49.00%		49.00%
Income from Equity Method Investments	$ 231,000	326,000	$ 484,000	431,000
Equity Method Investment	$ 13,562,000		$ 13,562,000		13,963,000
Luminis [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	19.00%		19.00%
Income from Equity Method Investments	$ 297,000	57,000	$ 297,000	57,000
Equity Method Investment	6,296,000		6,296,000		5,999,000
Equity Method Investments [Member]
Schedule of Investments [Line Items]
Amortization of Intangible Assets	223,000	$ 391,000	445,000	$ 781,000
G5 [Member]
Schedule of Investments [Line Items]
Held-to-maturity Securities	9,592,000		$ 9,592,000		10,995,000
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	776,000		$ 776,000		833,000
Cash	765,000		765,000
Marketable Securities	11,000		11,000
Glisco II [Member]
Schedule of Investments [Line Items]
Distribution Made to Limited Partner, Cash Distributions Paid			2,011,000
Trilantic IV [Member]
Schedule of Investments [Line Items]
Distribution Made to Limited Partner, Cash Distributions Paid			194,000
Investment					1,178,000
Trilantic V [Member]
Schedule of Investments [Line Items]
Distribution Made to Limited Partner, Cash Distributions Paid			997,000
Investment	417,000		417,000		4,513,000
Capital Commitment	5,000,000		5,000,000
Unfunded Commitments for Capital Contributions	683,000		683,000
Trilantic [Member]
Schedule of Investments [Line Items]
Issued LP Units (in shares)						500,000
Limited Partnership Investment						$ 16,090,000
Investment	9,982,000		9,982,000		10,399,000
Participation in Successor Funds	12,000,000		12,000,000
Glisco [Member]
Schedule of Investments [Line Items]
Equity Securities without Readily Determinable Fair Value, Amount	$ 2,082,000		$ 2,082,000		$ 2,172,000