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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives asset, Notional	$ 505,613		$ 505,613		$ 297,727
Derivatives asset, Fair Value	2,901		2,901		1,663
Derivative Liability, Notional Amount	4,663,277		4,663,277		4,756,631
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	4,130		4,130		2,024
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(378)	$ (2,327)	376	$ (3,839)
Commodity Contracts [Member] | Future [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability, Notional Amount	3,702,000		3,702,000		4,059,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	853		853		166
Commodity Contracts [Member] | Principal Transaction Revenue [Member] | Future [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(8)	(1,306)	234	(3,061)
Other Contracts [Member] | To Be Announced Security [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives asset, Notional	314,255		314,255		169,500
Derivatives asset, Fair Value	1,257		1,257		332
Derivative Liability, Notional Amount	309,555		309,555		169,500
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	1,180		1,180		289
Other Contracts [Member] | ARS Purchase Commitments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives asset, Notional					6,654
Derivatives asset, Fair Value					849
Other Contracts [Member] | Tba Sale Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives asset, Notional	62,358		62,358		121,573
Derivatives asset, Fair Value	1,644		1,644		482
Other Contracts [Member] | TBA Purchase Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability, Notional Amount	62,358		62,358		121,573
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	1,835		1,835		923
Other Contracts [Member] | Forward Start Repurchase Agreement [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability, Notional Amount					382,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral					0
Other Contracts [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability, Notional Amount	14,964		14,964		24,358
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	254		254		645
Other Contracts [Member] | Foreign Exchange Forward [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability, Notional Amount	400		400		200
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	8		8		$ 1
Other Contracts [Member] | Principal Transaction Revenue [Member] | Tba [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	204	(4)	77	(13)
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)	(773)	(212)	(458)	2,138
Other Contracts [Member] | Other [Member] | Tba Sale Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(1,644)	(228)	(1,860)	(8,129)
Other Contracts [Member] | Other [Member] | TBA Purchase Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	1,835		2,371
Other Contracts [Member] | Other [Member] | Foreign Exchange Forward [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ 8	(6)	$ 12	11
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)		$ 5,215