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	Sep. 30, 2018	Dec. 31, 2017
Financial instruments and other inventory positions owned:
Equity securities	$ 21,473	$ 51,896
Convertible securities	126,641	74,456
Fixed income securities	42,976	30,145
Taxable securities	41,766	67,699
Tax-exempt securities	226,702	744,241
Short-term securities	150,090	62,251
Mortgage-backed securities	16	481
U.S. government agency securities	173,477	317,318
U.S. government securities	1,561	9,317
Derivative contracts	13,418	25,573
Total financial instruments and other inventory positions owned	798,120	1,383,377
Financial instruments and other inventory positions sold, but not yet purchased:
Equity securities	116,712	101,517
Fixed income securities	31,187	30,292
U.S. government agency securities	37,245	49,077
U.S. government securities	65,205	213,312
Derivative contracts	4,236	5,029
Total financial instruments and other inventory positions sold, but not yet purchased	254,585	399,227
Financial instruments and other inventory positions owned and pledged as collateral	$ 162,227	$ 720,047