Derivative Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Derivative [Line Items]
Net (losses) on derivative instruments	$ (6,055,000)	$ (5,521,000)
Cash collateral in brokerage accounts	(11,931,000)		(13,241,000)
Cash collateral payable to trade counterparties	(1,078,000)		(990,000)
Losses on equity options activity	32,800,000	60,700,000
Exchange Traded Options [Member] | Long [Member]
Derivative [Line Items]
Derivative Assets	6,800,000		22,300,000
Exchange Traded Options [Member] | Short [Member]
Derivative [Line Items]
Derivative Liabilities	5,600,000		7,100,000
Exchange-Traded Futures [Member]
Derivative [Line Items]
Net (losses) on derivative instruments	(4,987,000)	(498,000)
Currency Forwards [Member]
Derivative [Line Items]
Net (losses) on derivative instruments	3,452,000	(1,666,000)
Interest Rate Swaps [Member]
Derivative [Line Items]
Net (losses) on derivative instruments	(745,000)	(793,000)
Credit Default Swaps [Member]
Derivative [Line Items]
Net (losses) on derivative instruments	(57,000)	(5,000)
Option Swaps [Member]
Derivative [Line Items]
Net (losses) on derivative instruments	(17,000)	(35,000)
Total Return Swaps [Member]
Derivative [Line Items]
Net (losses) on derivative instruments	(3,701,000)	(2,524,000)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Value	674,296,000		508,405,000
Derivative Assets	8,551,000		6,787,000
Derivative Liabilities	9,773,000		9,392,000
Not Designated as Hedging Instrument [Member] | Exchange-Traded Futures [Member]
Derivative [Line Items]
Notional Value	143,141,000		149,863,000
Derivative Assets	360,000		571,000
Derivative Liabilities	3,011,000		2,438,000
Not Designated as Hedging Instrument [Member] | Currency Forwards [Member]
Derivative [Line Items]
Notional Value	204,894,000		149,282,000
Derivative Assets	1,948,000		1,782,000
Derivative Liabilities	963,000		333,000
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Notional Value	59,126,000		50,591,000
Derivative Assets	1,909,000		1,507,000
Derivative Liabilities	3,045,000		2,679,000
Not Designated as Hedging Instrument [Member] | Credit Default Swaps [Member]
Derivative [Line Items]
Notional Value	165,028,000		32,745,000
Derivative Assets	3,881,000		1,432,000
Derivative Liabilities	293,000		110,000
Not Designated as Hedging Instrument [Member] | Option Swaps [Member]
Derivative [Line Items]
Notional Value	11,000		11,000
Derivative Assets	11,000		107,000
Derivative Liabilities	0		88,000
Not Designated as Hedging Instrument [Member] | Total Return Swaps [Member]
Derivative [Line Items]
Notional Value	102,096,000		125,913,000
Derivative Assets	442,000		1,388,000
Derivative Liabilities	$ 2,461,000		$ 3,744,000