Fair Value Measurements - Fair Value Amounts of Derivative Instruments and their Effect on Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ 179	$ 12,629	$ 1,700	$ 3,935
Commodity Contracts [Member] | Future [Member] | Principal Transaction Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(641)	(657)	(2,121)	(1,329)
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)	(237)	691	(132)	1,086
Other Contracts [Member] | Other [Member] | Tba Sale Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	7,531	4,596	5,912	(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)	(6,503)	8,059	(1,977)	7,973
Other Contracts [Member] | Foreign Exchange Forward [Member] | Other [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	28		31
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)	$ 1	$ (60)	$ (13)	$ (24)