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		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ (383)	$ (7,471)	$ 3,552	$ (5,485)
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)	20	(356)	(1,040)	446
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)	(119)	(235)	(272)	(287)
Commodity Contracts [Member] | general collateral futures [Member] | Principal Transaction Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(4)		(2)
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)	(2)	(36)	(120)	35
Other Contracts [Member] | Tba Sale Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	3,461
Other Contracts [Member] | Other [Member] | Tba Sale Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)		(6,518)	(310)
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)	(4,372)		3,601
Other Contracts [Member] | To Be Announced Security [Member] | Principal Transaction Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	156	(243)	132	12
Other Contracts [Member] | To Be Announced Security [Member] | Other [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)				(5,926)
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)	$ 477	$ (83)	$ 1,563	$ 235