Fair Value Measurements - Fair Value Amounts of Derivative Instruments and their Effect on Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ (3,564)	$ (383)	$ (1,309)	$ 3,552
Commodity Contracts [Member] | Future [Member] | Principal Transaction Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(1,107)	(105)	(2,673)	(1,434)
Other Contracts [Member] | Principal Transaction Revenue [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	557	477	425	1,563
Other Contracts [Member] | Other [Member] | Tba Sale Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(9,242)	3,461	(3,330)	(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)	6,244	(4,372)	4,267	3,601
Other Contracts [Member] | Foreign Exchange Forward [Member] | Other [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(5)		26
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)	$ (11)	$ 156	$ (24)	$ 132