Related Party Transactions, Compensation and Other (Details) - USD ($)			3 Months Ended	9 Months Ended	12 Months Ended
	Nov. 21, 2017	Sep. 30, 2017	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 29, 2017	Dec. 27, 2017	Aug. 15, 2016	Nov. 30, 2015	Jul. 02, 2015	May 31, 2006
Compensation [Abstract]
Percentage payout of revenues or net operating contribution to the portfolio managers and marketing staff who introduce, service or generate private wealth management business			40.00%		40.00%
Payable to affiliates			$ 855,000		$ 855,000	$ 1,412,000
Other [Abstract]
Deposited transaction amount					328,000	353,000	$ 432,000
Institutional research services					$ 2,300,000	1,500,000	700,000
Notice period of agreement between related parties					30 days
Term of agreement between related parties					12 months
Annual costs of setting up escrow account paid					$ 55,000
Period of interest included in initial deposit					6 months
AC 4% PIK Note [Member]
Other [Abstract]
Face value of debt			$ 50,000,000		$ 50,000,000							$ 250,000,000
Debt instrument, interest rate			4.00%		4.00%							4.00%
Debt instrument, maturity date					Nov. 30, 2020
Interest expenses					$ 3,000,000	7,700,000	800,000
Loan from GGCP [Member]
Other [Abstract]
Face value of debt			$ 35,000,000		$ 35,000,000
Debt instrument, maturity date					Dec. 28, 2016
Interest expenses						415,000
Loan from AC [Member]
Other [Abstract]
Face value of debt										$ 15,000,000
Debt instrument, interest rate										1.60%
Debt instrument, maturity date					Feb. 28, 2018
Interest expenses					$ 4,000
Convertible Notes [Member]
Other [Abstract]
Face value of debt											$ 110,000,000
Debt instrument, interest rate			4.50%		4.50%						4.50%
Debt instrument, maturity date			Aug. 15, 2021		Aug. 15, 2021
Percentage of assets owned in escrow account	47.00%	53.00%
Cash deposited into escrow account	$ 53,000,000	$ 60,000,000
GCI [Member] | 5.5% Demand Loan [Member]
Other [Abstract]
Face value of debt								$ 16,000,000
Debt instrument, interest rate								5.50%
Interest expenses							900,000	$ 1,000,000
Fund Advisor [Member]
Other [Abstract]
Institutional research services					$ 2,300,000	1,500,000	800,000
Significant Shareholder [Member] | Common Class A [Member]
Other [Abstract]
Shares included in the Exchange and Standstill agreement (in shares)													1,705,974
Significant Shareholder [Member] | Common Class B [Member]
Other [Abstract]
Shares included in the Exchange and Standstill agreement (in shares)														2,071,635
GGCP [Member]
Other [Abstract]
Percentage of assets owned in escrow account	47.00%			53.00%
Cash deposited into escrow account	$ 53,100,000			$ 60,000,000
GGCP [Member] | Convertible Notes [Member]
Other [Abstract]
Additional amount paid to repurchase debt	$ 1,400,000
GBL and Teton [Member]
Other [Abstract]
Flat sub-administration agreement percentage on mutual funds					0.20%
Administration fees percentage on first tier of net assets					0.20%
First tier net assets maximum					$ 370,000,000
Administration fees percentage on second tier of net assets					0.12%
Second tier net assets maximum					$ 630,000,000
Administration fees percentage on third tier of net assets					0.10%
Third tier net assets minimum					$ 1,000,000,000
Monthly administrative service fee					4,167	18,750	25,000
Administrative and management services					$ 2,100,000	$ 2,000,000	$ 2,200,000
Certain Executive Officers and Employees [Member]
Compensation [Abstract]
Payable to affiliates									$ 11,700,000
Certain Executive Officers and Employees [Member] | Due on January 31, 2018 [Member]
Compensation [Abstract]
Payable to affiliates									5,500,000
Certain Executive Officers and Employees [Member] | Due on February 28, 2018 [Member]
Compensation [Abstract]
Payable to affiliates									$ 6,200,000