Fair Value of Financial Instruments - Valuation of Financial Instruments by Pricing Observability Levels (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Assets
Equity securities	$ 5,780,000	$ 1,349,000
Convertible securities	150,979,000	146,088,000
Fixed income securities	21,556,000	18,432,000
Taxable securities	23,330,000	6,267,000
Tax-exempt securities	117,212,000	67,944,000
Short-term securities	56,486,000	28,592,000
Mortgage-backed securities	13,000	13,000
U.S. government agency securities	51,838,000	9,146,000
U.S. government securities	48,000	100,275,000
Derivative contracts	23,573,000	23,446,000
Total financial instruments and other inventory positions owned	450,815,000	401,552,000
Liabilities
Equity securities	62,407,000	105,190,000
Fixed income securities	5,468,000	18,789,000
U.S. government securities	58,277,000	21,669,000
Derivative contracts	5,397,000	5,382,000
Total financial instruments and other inventory positions sold, but not yet purchased	131,549,000	151,030,000
Securities posted as collateral	0	0
Level III
Assets
Total assets	158,800,000	153,300,000
Measured on a recurring basis
Assets
Equity securities	5,780,000	1,349,000
Convertible securities	150,979,000	146,088,000
Fixed income securities	21,556,000	18,432,000
Taxable securities	23,330,000	6,267,000
Tax-exempt securities	117,212,000	67,944,000
Short-term securities	56,486,000	28,592,000
Mortgage-backed securities	13,000	13,000
U.S. government agency securities	51,838,000	9,146,000
U.S. government securities	48,000	100,275,000
Derivative contracts	23,573,000	23,446,000
Derivative contracts - counterparty and collateral netting	(144,225,000)	(209,670,000)
Total financial instruments and other inventory positions owned	450,815,000	401,552,000
Cash equivalents	524,920,000	468,091,000
Investments at fair value (2)	223,453,000	174,849,000
Total assets	1,199,188,000	1,044,492,000
Liabilities
Equity securities	62,407,000	105,190,000
Fixed income securities	5,468,000	18,789,000
U.S. government securities	58,277,000	21,669,000
Derivative contracts	5,397,000	5,382,000
Derivative contracts - counterparty and collateral netting	(154,364,000)	(222,061,000)
Total financial instruments and other inventory positions sold, but not yet purchased	131,549,000	151,030,000
Measured on a recurring basis | Investments attributable to noncontrolling interests
Assets
Investments at fair value (2)	136,300,000	94,900,000
Measured on a recurring basis | Level I
Assets
Equity securities	692,000	330,000
U.S. government securities	48,000	100,275,000
Total financial instruments and other inventory positions owned	740,000	100,605,000
Cash equivalents	524,920,000	468,091,000
Investments at fair value (2)	21,330,000	16,496,000
Total assets	546,990,000	585,192,000
Liabilities
Equity securities	60,902,000	102,013,000
U.S. government securities	58,277,000	21,669,000
Total financial instruments and other inventory positions sold, but not yet purchased	119,179,000	123,682,000
Measured on a recurring basis | Level II
Assets
Equity securities	5,088,000	1,019,000
Convertible securities	150,979,000	146,088,000
Fixed income securities	21,556,000	18,432,000
Taxable securities	23,330,000	6,267,000
Tax-exempt securities	116,931,000	67,944,000
Short-term securities	56,486,000	28,592,000
U.S. government agency securities	51,838,000	9,146,000
Derivative contracts	166,682,000	232,846,000
Total financial instruments and other inventory positions owned	592,890,000	510,334,000
Investments at fair value (2)	44,718,000	5,358,000
Total assets	637,608,000	515,692,000
Liabilities
Equity securities	1,505,000	3,177,000
Fixed income securities	5,468,000	18,789,000
Derivative contracts	158,900,000	223,737,000
Total financial instruments and other inventory positions sold, but not yet purchased	165,873,000	245,703,000
Measured on a recurring basis | Level III
Assets
Tax-exempt securities	281,000
Mortgage-backed securities	13,000	13,000
Derivative contracts	1,116,000	270,000
Total financial instruments and other inventory positions owned	1,410,000	283,000
Investments at fair value (2)	157,405,000	152,995,000
Total assets	158,815,000	153,278,000
Liabilities
Derivative contracts	861,000	3,706,000
Total financial instruments and other inventory positions sold, but not yet purchased	$ 861,000	$ 3,706,000