Financial Instruments and Other Inventory Positions Owned and Financial Instruments and Other Inventory Positions Sold, but Not Yet Purchased - Schedule of Gross Fair Market Value and Total Absolute Notional Contract Amount (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional amount	$ 2,248,684	$ 2,308,215
Financial instruments and other inventory positions owned
Derivative [Line Items]
Derivative Assets	266,534	209,127
Financial instruments and other inventory positions sold, but not yet purchased
Derivative [Line Items]
Derivative Liabilities	261,718	200,167
Customer matched-book | Interest rate
Derivative [Line Items]
Notional amount	2,023,391	2,197,340
Customer matched-book | Interest rate | Financial instruments and other inventory positions owned
Derivative [Line Items]
Derivative Assets	264,559	209,119
Customer matched-book | Interest rate | Financial instruments and other inventory positions sold, but not yet purchased
Derivative [Line Items]
Derivative Liabilities	253,830	198,315
Trading securities | Interest rate
Derivative [Line Items]
Notional amount	209,075	110,875
Trading securities | Interest rate | Financial instruments and other inventory positions owned
Derivative [Line Items]
Derivative Assets	1,975	8
Trading securities | Interest rate | Financial instruments and other inventory positions sold, but not yet purchased
Derivative [Line Items]
Derivative Liabilities	6,502	1,852
Trading securities | Equity options
Derivative [Line Items]
Notional amount	16,218	0
Trading securities | Equity options | Financial instruments and other inventory positions owned
Derivative [Line Items]
Derivative Assets	0	0
Trading securities | Equity options | Financial instruments and other inventory positions sold, but not yet purchased
Derivative [Line Items]
Derivative Liabilities	$ 1,386	$ 0