Derivative Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ (5,521,000)	$ (7,083,000)
Cash collateral payable to trade counterparties	2,700,000		1,400,000
Return of cash collateral into brokerage accounts	9,100,000		8,900,000
Exchange-Traded Futures [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	(498,000)	(3,931,000)
Currency Forwards [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	(1,666,000)	176,000
Interest Rate Swaps [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	(793,000)	283,000
Credit Default Swaps [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	(5,000)	(112,000)
Option Swaps [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	(35,000)	(124,000)
Total Return Swaps [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	(2,524,000)	(3,375,000)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Value	469,648,000		383,655,000
Asset Derivatives	3,652,000		3,285,000
Liability Derivatives	7,014,000		6,311,000
Not Designated as Hedging Instrument [Member] | Exchange-Traded Futures [Member]
Derivative [Line Items]
Notional Value	70,587,000		63,107,000
Asset Derivatives	144,000		289,000
Liability Derivatives	2,348,000		2,542,000
Not Designated as Hedging Instrument [Member] | Currency Forwards [Member]
Derivative [Line Items]
Notional Value	192,570,000		111,774,000
Asset Derivatives	566,000		576,000
Liability Derivatives	1,678,000		927,000
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Notional Value	72,452,000		81,253,000
Asset Derivatives	1,021,000		1,149,000
Liability Derivatives	951,000		573,000
Not Designated as Hedging Instrument [Member] | Credit Default Swaps [Member]
Derivative [Line Items]
Notional Value	52,065,000		42,270,000
Asset Derivatives	810,000		696,000
Liability Derivatives	363,000		126,000
Not Designated as Hedging Instrument [Member] | Option Swaps [Member]
Derivative [Line Items]
Notional Value	232,000		144,000
Asset Derivatives	188,000		87,000
Liability Derivatives	101,000		86,000
Not Designated as Hedging Instrument [Member] | Total Return Swaps [Member]
Derivative [Line Items]
Notional Value	81,742,000		85,107,000
Asset Derivatives	923,000		488,000
Liability Derivatives	$ 1,573,000		$ 2,057,000