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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives asset, Notional	$ 46,190		$ 46,190		$ 297,727
Derivatives asset, Fair Value	1,339		1,339		1,663
Derivative Liability, Notional Amount	3,514,485		3,514,485		4,756,631
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	2,169		2,169		2,024
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(138)	$ 640	(758)	$ (3,199)
Commodity Contracts [Member] | Future [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability, Notional Amount	3,452,000		3,452,000		4,059,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	663		663		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)	(20)	733	214	(2,328)
Other Contracts [Member] | Other TBA Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives asset, Notional	44,739		44,739		121,573
Derivatives asset, Fair Value	1,250		1,250		482
Derivative Liability, Notional Amount	44,739		44,739		121,573
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	1,130		1,130		923
Other Contracts [Member] | To Be Announced Security [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives asset, Notional	1,451		1,451		169,500
Derivatives asset, Fair Value	89		89		332
Derivative Liability, Notional Amount	2,223		2,223		169,500
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	100		100		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] | 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	15,523		15,523		24,358
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	276		276		645
Other Contracts [Member] | Foreign Exchange Forward [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability, Notional Amount					200
Derivative Liability, Fair Value, Amount Not Offset Against Collateral					$ 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)	(26)	32	(184)	19
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)	(22)	(140)	(480)	1,998
Other Contracts [Member] | Other [Member] | Other TBA Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ (70)	(39)	(320)	(8,168)
Other Contracts [Member] | Other [Member] | Foreign Exchange Forward [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)		1	$ 12	12
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)		$ 53		$ 5,268