Investments - Additional Information (Details) - USD ($)		3 Months Ended								12 Months Ended
	Nov. 30, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 31, 2017	Dec. 31, 2010
Schedule of Investments [Line Items]
Income from Equity Method Investments		$ 3,331,000	$ 1,827,000	$ 2,070,000	$ 1,610,000	$ 2,512,000	$ 1,178,000	$ 1,664,000	$ 1,287,000	$ 8,838,000	$ 6,641,000	$ 6,050,000
Amortization of Intangible Assets										9,793,000	11,640,000	17,458,000
Marketable Securities		65,032,000				66,487,000				65,032,000	66,487,000
Net Realized and Unrealized Gains (Losses) on Private Equity Fund Investments, Including Performance Fees										(915,000)	7,616,000	5,086,000
Previously Received Carried Interest Subject to Repayment		0								0
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets		8,730,000				9,889,000				8,730,000	9,889,000
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		10,996,000				12,232,000				10,996,000	12,232,000
Unfunded Commitments for Capital Contributions		3,408,000								3,408,000
Cost Method Investments		1,079,000				$ 1,079,000				1,079,000	1,079,000
Investment Banking [Member]
Schedule of Investments [Line Items]
Income from Equity Method Investments										277,000	1,370,000	978,000
Investment Management [Member]
Schedule of Investments [Line Items]
Income from Equity Method Investments										8,561,000	5,271,000	5,072,000
G5 [Member]
Schedule of Investments [Line Items]
Income from Equity Method Investments										(144,000)	1,154,000	662,000
Held-to-maturity Securities		$ 10,995,000								10,995,000
G5 [Member] | Investment Banking [Member]
Schedule of Investments [Line Items]
Fair Value Inputs, Discount Rate (as a percent)				17.50%
Compound Annual Growth Rate, Revenue (as a percent)				5.00%
G5 [Member] | Special Charges [Member] | Investment Banking [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Other than Temporary Impairment										14,400,000
ABS [Member]
Schedule of Investments [Line Items]
Income from Equity Method Investments										$ 7,990,000	4,913,000	5,388,000
Equity Method Investment, Ownership Percentage		45.00%								45.00%
Atalanta Sosnoff [Member]
Schedule of Investments [Line Items]
Fair Value Inputs, Discount Rate (as a percent)	15.50%
Compound Annual Growth Rate, Revenue (as a percent)	6.00%
Equity Method Investment	$ 14,730,000
Income from Equity Method Investments										$ 493,000	$ 574,000
Equity Method Investment, Ownership Percentage		49.00%				49.00%				49.00%	49.00%
Atalanta Sosnoff [Member] | Special Charges [Member] | Investment Management [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Other than Temporary Impairment											$ 8,100,000
Luminis [Member]
Schedule of Investments [Line Items]
Income from Equity Method Investments										$ 499,000
Equity Method Investment, Ownership Percentage		19.00%								19.00%
Glisco II [Member]
Schedule of Investments [Line Items]
Distribution Made to Limited Partner, Cash Distributions Paid										$ 2,106,000
Trilantic [Member]
Schedule of Investments [Line Items]
Issued LP Units (in shares)														500,000
Limited Partnership Investment														$ 16,090,000
Participation in Successor Funds		$ 12,000,000								12,000,000
Trilantic V [Member]
Schedule of Investments [Line Items]
Distribution Made to Limited Partner, Cash Distributions Paid										2,311,000
Trilantic IV [Member]
Schedule of Investments [Line Items]
Distribution Made to Limited Partner, Cash Distributions Paid											3,320,000
Glisco [Member]
Schedule of Investments [Line Items]
Percent Ownership of Carried Interest							20.00%
Cost Method Investments		2,172,000				$ 2,463,000	$ 2,519,000			$ 2,172,000	2,463,000
Variable Interest Entity, Not Primary Beneficiary [Member] | Minimum [Member] | ECP II [Member]
Schedule of Investments [Line Items]
Percent Ownership of Carried Interest										8.00%
Variable Interest Entity, Not Primary Beneficiary [Member] | Maximum [Member] | ECP II [Member]
Schedule of Investments [Line Items]
Percent Ownership of Carried Interest										9.00%
Equity Method Investments [Member]
Schedule of Investments [Line Items]
Amortization of Intangible Assets										$ 1,505,000	3,533,000	$ 2,484,000
Trilantic [Member]
Schedule of Investments [Line Items]
Investment		10,399,000				11,632,000				10,399,000	11,632,000
Trilantic [Member] | Trilantic V [Member]
Schedule of Investments [Line Items]
Investment		1,233,000				3,280,000				1,233,000	3,280,000
Capital Commitment		5,000,000								5,000,000
Unfunded Commitments for Capital Contributions		1,082,000								1,082,000
Trilantic [Member] | Trilantic IV [Member]
Schedule of Investments [Line Items]
Investment						1,178,000					1,178,000
Other Equity Method Investments [Member]
Schedule of Investments [Line Items]
Equity Method Investment		59,856,000				79,717,000				59,856,000	79,717,000
Other Equity Method Investments [Member] | G5 [Member]
Schedule of Investments [Line Items]
Equity Method Investment		0				26,016,000				0	26,016,000		$ 11,555,000
Other Equity Method Investments [Member] | ABS [Member]
Schedule of Investments [Line Items]
Equity Method Investment		39,894,000				38,982,000				39,894,000	38,982,000
Other Equity Method Investments [Member] | Atalanta Sosnoff [Member]
Schedule of Investments [Line Items]
Equity Method Investment		13,963,000				14,719,000				13,963,000	14,719,000
Other Equity Method Investments [Member] | Luminis [Member]
Schedule of Investments [Line Items]
Equity Method Investment		5,999,000				0				5,999,000	0
Private Equity Funds [Member]
Schedule of Investments [Line Items]
Equity Method Investment		13,812,000				21,742,000				13,812,000	21,742,000
Private Equity Funds [Member] | ECP II [Member]
Schedule of Investments [Line Items]
Equity Method Investment		833,000				933,000				833,000	933,000
Cash		804,000								804,000
Marketable Securities		29,000								29,000
Private Equity Funds [Member] | Trilantic V [Member]
Schedule of Investments [Line Items]
Equity Method Investment		$ 6,116,000				$ 5,709,000				$ 6,116,000	$ 5,709,000