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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ (2,327)	$ 179	$ (3,839)	$ 1,700
Commodity Contracts [Member] | Future [Member] | Principal Transaction Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(1,306)	(641)	(3,061)	(2,121)
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)	(212)	(237)	2,138	(132)
Other Contracts [Member] | Other [Member] | Tba Sale Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(228)	7,531		5,912
Other Contracts [Member] | Other [Member] | Forward Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)			(8,129)
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)	(571)	(6,503)	5,215	(1,977)
Other Contracts [Member] | Foreign Exchange Forward [Member] | Other [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(6)	28	11	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)	$ (4)	$ 1	$ (13)	$ (13)