Financial Instruments and Other Inventory Positions Owned and Financial Instruments and Other Inventory Positions Sold, but Not Yet Purchased - Additional Information (Details) - Maximum risk of loss $ in Millions	Mar. 31, 2020 USD ($)
Counterparties not required to post collateral
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Uncollateralized credit exposure	$ 27.3
Notional contract amount	173.0
One unnamed financial institutional not required to post collateral
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Uncollateralized credit exposure	$ 23.2