Financial Instruments and Other Inventory Positions Owned and Financial Instruments and Other Inventory Positions Sold, but Not Yet Purchased - Total Absolute Notional Contract Amount (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative Assets (1)	$ 452,759	$ 454,011
Derivative Liabilities (2)	433,509	438,846
Notional Amount	5,008,844	5,444,728
Interest rate | Customer matched-book
Derivative [Line Items]
Notional Amount	4,653,035	4,860,302
Interest rate | Customer matched-book | Financial instruments and other inventory positions owned
Derivative [Line Items]
Derivative Assets (1)	447,226	447,987
Interest rate | Customer matched-book | Financial instruments and other inventory positions sold, but not yet purchased
Derivative [Line Items]
Derivative Liabilities (2)	425,506	425,227
Interest rate | Trading securities
Derivative [Line Items]
Notional Amount	241,850	297,250
Interest rate | Trading securities | Financial instruments and other inventory positions owned
Derivative [Line Items]
Derivative Assets (1)	0	140
Interest rate | Trading securities | Financial instruments and other inventory positions sold, but not yet purchased
Derivative [Line Items]
Derivative Liabilities (2)	6,512	8,242
Credit default swap index | Trading securities
Derivative [Line Items]
Notional Amount	105,538	267,796
Credit default swap index | Trading securities | Financial instruments and other inventory positions owned
Derivative [Line Items]
Derivative Assets (1)	5,366	5,808
Credit default swap index | Trading securities | Financial instruments and other inventory positions sold, but not yet purchased
Derivative [Line Items]
Derivative Liabilities (2)	1,347	5,188
Equity option | Trading securities
Derivative [Line Items]
Notional Amount	8,421	19,380
Equity option | Trading securities | Financial instruments and other inventory positions owned
Derivative [Line Items]
Derivative Assets (1)	167	76
Equity option | Trading securities | Financial instruments and other inventory positions sold, but not yet purchased
Derivative [Line Items]
Derivative Liabilities (2)	$ 144	$ 189