REVENUE FROM CONTRACTS WITH CUSTOMERS - Schedule of revenues from contracts with customers (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	$ 199,701	$ 289,251
Interest income - Loans and securities lending	61,426	36,920
Trading (losses) gains on investments	(79,328)	256,216
Fair value adjustment on loans	10,938	10,726
Other	12,852	7,046
Total revenues	205,589	600,159
Sale of goods	1,878	6,828
Capital Markets Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	56,118	165,496
Interest income - Loans and securities lending	61,426	36,920
Trading (losses) gains on investments	(79,850)	253,777
Fair value adjustment on loans	10,938	10,726
Other	12,966	5,483
Total revenues	61,598	472,402
Wealth Management Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	77,071	63,979
Interest income - Loans and securities lending	0	0
Trading (losses) gains on investments	522	2,356
Fair value adjustment on loans	0	0
Other	(114)	1,563
Total revenues	77,479	67,898
Auction and Liquidation Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	3,355	13,450
Interest income - Loans and securities lending	0	0
Trading (losses) gains on investments	0	0
Fair value adjustment on loans	0	0
Other	0	0
Total revenues	3,355	13,450
Sale of goods		6,092
Financial Consulting Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	25,936	21,409
Interest income - Loans and securities lending	0	0
Trading (losses) gains on investments	0	0
Fair value adjustment on loans	0	0
Other	0
Total revenues	25,936	21,409
Principal Investments- Communications and Other Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	32,664	20,529
Interest income - Loans and securities lending	0	0
Trading (losses) gains on investments	0	0
Fair value adjustment on loans	0	0
Other	0	0
Total revenues	32,664	20,529
Sale of goods		736
Brands Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	4,557	4,388
Interest income - Loans and securities lending	0	0
Trading (losses) gains on investments		83
Fair value adjustment on loans	0	0
Other	0	0
Total revenues	4,557	4,471
Corporate finance, consulting and investment banking fees
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	58,643	160,496
Corporate finance, consulting and investment banking fees | Capital Markets Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	41,673	147,069
Corporate finance, consulting and investment banking fees | Wealth Management Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Corporate finance, consulting and investment banking fees | Auction and Liquidation Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Corporate finance, consulting and investment banking fees | Financial Consulting Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	16,970	13,427
Corporate finance, consulting and investment banking fees | Principal Investments- Communications and Other Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Corporate finance, consulting and investment banking fees | Brands Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Wealth and asset management fees
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	66,622	53,394
Wealth and asset management fees | Capital Markets Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	2,400	2,883
Wealth and asset management fees | Wealth Management Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	64,222	50,511
Wealth and asset management fees | Auction and Liquidation Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Wealth and asset management fees | Financial Consulting Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Wealth and asset management fees | Principal Investments- Communications and Other Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Wealth and asset management fees | Brands Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Commissions, fees and reimbursed expenses
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	37,215	44,051
Commissions, fees and reimbursed expenses | Capital Markets Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	12,045	15,544
Commissions, fees and reimbursed expenses | Wealth Management Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	12,849	13,468
Commissions, fees and reimbursed expenses | Auction and Liquidation Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	3,355	7,057
Commissions, fees and reimbursed expenses | Financial Consulting Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	8,966	7,982
Commissions, fees and reimbursed expenses | Principal Investments- Communications and Other Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Commissions, fees and reimbursed expenses | Brands Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Subscription services
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	27,813	17,244
Subscription services | Capital Markets Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Subscription services | Wealth Management Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Subscription services | Auction and Liquidation Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Subscription services | Financial Consulting Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Subscription services | Principal Investments- Communications and Other Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	27,813	17,244
Subscription services | Brands Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Service contract revenues
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	301
Service contract revenues | Capital Markets Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Service contract revenues | Wealth Management Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Service contract revenues | Auction and Liquidation Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	301
Service contract revenues | Financial Consulting Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Service contract revenues | Principal Investments- Communications and Other Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Service contract revenues | Brands Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Advertising, licensing and other
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	9,408	13,765
Advertising, licensing and other | Capital Markets Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Advertising, licensing and other | Wealth Management Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Advertising, licensing and other | Auction and Liquidation Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	6,092
Advertising, licensing and other | Financial Consulting Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	0	0
Advertising, licensing and other | Principal Investments- Communications and Other Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	4,851	3,285
Sale of goods	1,878
Advertising, licensing and other | Brands Segment
Revenue from External Customer [Line Items]
Total revenues from contracts with customers	$ 4,557	$ 4,388