Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Other sources of revenue:
Total revenues		$ 165,237	$ 180,063	$ 164,684	$ 142,128	$ 102,031	$ 99,681	$ 125,501	$ 95,778	$ 652,112		$ 422,991		$ 322,176
Revenue [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										458,486		384,648
Other sources of revenue:
Total revenues										652,112		422,991
Capital Markets [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										292,232		236,723
Other sources of revenue:
Total revenues										485,858		275,066
Principal Investments - United Online And MagicJack [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										100,862		54,234
Other sources of revenue:
Total revenues										100,862		54,234
Auction and Liquidation Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										22,516		54,986
Other sources of revenue:
Total revenues										22,516		54,986
Valuation and Appraisal Reportable Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										38,821		38,705
Other sources of revenue:
Total revenues										38,821		38,705
Brands [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										4,055
Other sources of revenue:
Total revenues										4,055
Other [Member]
Other sources of revenue:
Total revenues										20,918		16,195
Other [Member] | Capital Markets [Member]
Other sources of revenue:
Total revenues										20,918		16,195
Other [Member] | Principal Investments - United Online And MagicJack [Member]
Other sources of revenue:
Total revenues
Other [Member] | Auction and Liquidation Segment [Member]
Other sources of revenue:
Total revenues
Other [Member] | Valuation and Appraisal Reportable Segment [Member]
Other sources of revenue:
Total revenues
Other [Member] | Brands [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Trading gains on investments [Member]
Other sources of revenue:
Total revenues										95,487		(16,129)
Trading gains on investments [Member] | Capital Markets [Member]
Other sources of revenue:
Total revenues										95,487		(16,129)
Trading gains on investments [Member] | Principal Investments - United Online And MagicJack [Member]
Other sources of revenue:
Total revenues
Trading gains on investments [Member] | Auction and Liquidation Segment [Member]
Other sources of revenue:
Total revenues
Trading gains on investments [Member] | Valuation and Appraisal Reportable Segment [Member]
Other sources of revenue:
Total revenues
Trading gains on investments [Member] | Brands [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Corporate finance, consulting and investment banking fees [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										166,951		117,978
Corporate finance, consulting and investment banking fees [Member] | Capital Markets [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										166,951		117,978
Corporate finance, consulting and investment banking fees [Member] | Principal Investments - United Online And MagicJack [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Corporate finance, consulting and investment banking fees [Member] | Auction and Liquidation Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Corporate finance, consulting and investment banking fees [Member] | Valuation and Appraisal Reportable Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Corporate finance, consulting and investment banking fees [Member] | Brands [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Wealth and asset management fees [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										82,778		74,510
Wealth and asset management fees [Member] | Capital Markets [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										82,778		74,510
Wealth and asset management fees [Member] | Principal Investments - United Online And MagicJack [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Wealth and asset management fees [Member] | Auction and Liquidation Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Wealth and asset management fees [Member] | Valuation and Appraisal Reportable Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Wealth and asset management fees [Member] | Brands [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Commissions, fees and reimbursed expenses [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										131,173		119,190
Commissions, fees and reimbursed expenses [Member] | Capital Markets [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										42,503		44,235
Commissions, fees and reimbursed expenses [Member] | Principal Investments - United Online And MagicJack [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Commissions, fees and reimbursed expenses [Member] | Auction and Liquidation Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										49,849		36,250
Commissions, fees and reimbursed expenses [Member] | Valuation and Appraisal Reportable Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										38,821		38,705
Commissions, fees and reimbursed expenses [Member] | Brands [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Subscription services [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										82,088		42,887
Subscription services [Member] | Capital Markets [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Subscription services [Member] | Principal Investments - United Online And MagicJack [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										82,088		42,887
Subscription services [Member] | Auction and Liquidation Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Subscription services [Member] | Valuation and Appraisal Reportable Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Subscription services [Member] | Brands [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Service contract revenues [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										(31,553)		18,673
Service contract revenues [Member] | Capital Markets [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Service contract revenues [Member] | Principal Investments - United Online And MagicJack [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Service contract revenues [Member] | Auction and Liquidation Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										(31,553)		18,673
Service contract revenues [Member] | Valuation and Appraisal Reportable Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Service contract revenues [Member] | Brands [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers
Advertising, licensing and other [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										27,049	[1]	11,347	[2]
Advertising, licensing and other [Member] | Capital Markets [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers	[1]
Advertising, licensing and other [Member] | Principal Investments - United Online And MagicJack [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										18,774	[1]	11,410	[2]
Advertising, licensing and other [Member] | Auction and Liquidation Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										4,220	[1]	63	[2]
Advertising, licensing and other [Member] | Valuation and Appraisal Reportable Segment [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers											[1]		[2]
Advertising, licensing and other [Member] | Brands [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers										4,055	[1]		[2]
Interest income - Loans and securities lending [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers												38,277
Other sources of revenue:
Total revenues										77,221		38,277		$ 19,756
Interest income - Loans and securities lending [Member] | Capital Markets [Member]
Other sources of revenue:
Total revenues										77,221		38,277
Interest income - Loans and securities lending [Member] | Principal Investments - United Online And MagicJack [Member]
Other sources of revenue:
Total revenues
Interest income - Loans and securities lending [Member] | Auction and Liquidation Segment [Member]
Other sources of revenue:
Total revenues
Interest income - Loans and securities lending [Member] | Valuation and Appraisal Reportable Segment [Member]
Other sources of revenue:
Total revenues
Interest income - Loans and securities lending [Member] | Brands [Member]
Revenues from contracts with customers:
Total revenues from contracts with customers

[1] Includes sale of goods of $4,220 in Auction Liquidation and $3,715 in Principal Investments - United Online and magicJack.

[2] Includes sale of goods of $63 in Auction Liquidation and $575 in Principal Investments - United Online and magicJack.