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	Jun. 30, 2017 USD ($)
Counterparties not required to post collateral
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Uncollateralized credit exposure	$ 20.1
Notional contract amount	182.1
One unnamed financial institutional not required to post collateral
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Uncollateralized credit exposure	$ 15.5