Financial Instruments - Fair Value Amounts of Derivative Instruments and their Effect on Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ 12,629	$ 2,214	$ 3,935	$ 3,998
Commodity Contracts [Member] | U.S. Treasury Futures [Member] | Principal Transaction Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(637)	921	(1,060)	801
Commodity Contracts [Member] | Federal Funds Futures [Member] | Principal Transaction Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	7	(107)	(153)	(52)
Commodity Contracts [Member] | Euro Dollars Futures [Member] | Principal Transaction Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(27)		(116)	72
Other Contracts [Member] | Tba Sale Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	4,596
Other Contracts [Member] | Other [Member] | Tba Sale Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)		692	(3,771)
Other Contracts [Member] | Other [Member] | Interest Rate Lock Commitments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	8,059		7,973
Other Contracts [Member] | TBAs [Member] | Principal Transaction Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(60)	173	(24)	256
Other Contracts [Member] | TBAs [Member] | Other [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)				592
Other Contracts [Member] | Auction Rate Securities Purchase Commitment [Member] | Principal Transaction Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ 691	$ 535	$ 1,086	$ 2,329