Financial Instruments and Other Inventory Positions Owned and Financial Instruments and Other Inventory Positions Sold, but Not Yet Purchased - Schedule of Financial Instruments Owned and Financial Instruments Sold, but Not Yet Purchased by Type (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financial instruments and other inventory positions owned:
Equity securities	$ 2,827	$ 3,046
Convertible securities	117,915	146,406
Fixed income securities	84,651	28,176
Taxable securities	26,260	22,570
Tax-exempt securities	214,817	222,192
Short-term securities	63,406	67,901
Mortgage-backed securities	13	13
U.S. government agency securities	68,076	51,773
U.S. government securities	407	77,303
Derivative contracts	32,971	20,382
Total financial instruments and other inventory positions owned	611,343	639,762
Financial instruments and other inventory positions sold, but not yet purchased:
Equity securities	50,895	94,036
Fixed income securities	13,995	10,311
U.S. government agency securities	881	9,935
U.S. government securities	73,207	67,090
Derivative contracts	8,140	4,053
Total financial instruments and other inventory positions sold, but not yet purchased	147,118	185,425
Financial instruments and other inventory positions owned and pledged as collateral	$ 369,948	$ 205,674