Fair Value of Financial Instruments - Valuation of Financial Instruments by Pricing Observability Levels (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Assets
Equity securities	$ 2,827,000	$ 3,046,000
Convertible securities	117,915,000	146,406,000
Fixed income securities	84,651,000	28,176,000
Taxable securities	26,260,000	22,570,000
Tax-exempt securities	214,817,000	222,192,000
Short-term securities	63,406,000	67,901,000
Mortgage-backed securities	13,000	13,000
U.S. government agency securities	68,076,000	51,773,000
U.S. government securities	407,000	77,303,000
Derivative contracts	32,971,000	20,382,000
Total financial instruments and other inventory positions owned	611,343,000	639,762,000
Liabilities
Equity securities	50,895,000	94,036,000
Fixed income securities	13,995,000	10,311,000
U.S. government agency securities	881,000	9,935,000
U.S. government securities	73,207,000	67,090,000
Derivative contracts	8,140,000	4,053,000
Total financial instruments and other inventory positions sold, but not yet purchased	147,118,000	185,425,000
Securities posted as collateral	0	0
Level III
Assets
Total assets	122,700,000	132,400,000
Measured on a recurring basis
Assets
Equity securities	2,827,000	3,046,000
Convertible securities	117,915,000	146,406,000
Fixed income securities	84,651,000	28,176,000
Taxable securities	26,260,000	22,570,000
Tax-exempt securities	214,817,000	222,192,000
Short-term securities	63,406,000	67,901,000
Mortgage-backed securities	13,000	13,000
U.S. government agency securities	68,076,000	51,773,000
U.S. government securities	407,000	77,303,000
Derivative contracts	32,971,000	20,382,000
Derivative contracts - counterparty and collateral netting	(233,563,000)	(188,745,000)
Total financial instruments and other inventory positions owned	611,343,000	639,762,000
Cash equivalents	4,916,000	226,744,000
Investments at fair value	134,568,000	149,987,000
Total assets	750,827,000	1,016,493,000
Liabilities
Equity securities	50,895,000	94,036,000
Fixed income securities	13,995,000	10,311,000
U.S. government agency securities	881,000	9,935,000
U.S. government securities	73,207,000	67,090,000
Derivative contracts	8,140,000	4,053,000
Derivative contracts - counterparty and collateral netting	(253,578,000)	(196,114,000)
Total financial instruments and other inventory positions sold, but not yet purchased	147,118,000	185,425,000
Measured on a recurring basis | Level I
Assets
Equity securities	165,000	469,000
U.S. government securities	407,000	77,303,000
Total financial instruments and other inventory positions owned	572,000	77,772,000
Cash equivalents	4,916,000	226,744,000
Investments at fair value	13,838,000	17,658,000
Total assets	19,326,000	322,174,000
Liabilities
Equity securities	50,872,000	88,794,000
U.S. government securities	73,207,000	67,090,000
Derivative contracts	1,386,000
Total financial instruments and other inventory positions sold, but not yet purchased	125,465,000	155,884,000
Measured on a recurring basis | Level II
Assets
Equity securities	2,662,000	2,577,000
Convertible securities	117,915,000	146,406,000
Fixed income securities	84,651,000	28,176,000
Taxable securities	26,260,000	22,570,000
Tax-exempt securities	214,817,000	222,192,000
Short-term securities	63,406,000	67,901,000
U.S. government agency securities	68,076,000	51,773,000
Derivative contracts	264,559,000	209,119,000
Total financial instruments and other inventory positions owned	842,346,000	750,714,000
Total assets	842,346,000	750,714,000
Liabilities
Equity securities	23,000	5,242,000
Fixed income securities	13,995,000	10,311,000
U.S. government agency securities	881,000	9,935,000
Derivative contracts	254,775,000	198,604,000
Total financial instruments and other inventory positions sold, but not yet purchased	269,674,000	224,092,000
Measured on a recurring basis | Level III
Assets
Mortgage-backed securities	13,000	13,000
Derivative contracts	1,975,000	8,000
Total financial instruments and other inventory positions owned	1,988,000	21,000
Investments at fair value	120,730,000	132,329,000
Total assets	122,718,000	132,350,000
Liabilities
Derivative contracts	5,557,000	1,563,000
Total financial instruments and other inventory positions sold, but not yet purchased	5,557,000	1,563,000
Measured on a recurring basis | Level III | Investments attributable to noncontrolling interests
Assets
Investments at fair value	$ 66,500,000	$ 75,200,000