Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Notional Value	$ 610,582	$ 508,405
Asset Derivatives	14,569	6,787
Liability Derivatives	10,615	9,392
Exchange-Traded Futures [Member]
Derivative [Line Items]
Notional Value	147,181	149,863
Asset Derivatives	2,347	571
Liability Derivatives	168	2,438
Currency Forwards [Member]
Derivative [Line Items]
Notional Value	256,119	149,282
Asset Derivatives	4,145	1,782
Liability Derivatives	3,750	333
Interest Rate Swaps [Member]
Derivative [Line Items]
Notional Value	76,058	50,591
Asset Derivatives	4,579	1,507
Liability Derivatives	5,146	2,679
Credit Default Swaps [Member]
Derivative [Line Items]
Notional Value	27,244	32,745
Asset Derivatives	1,451	1,432
Liability Derivatives	357	110
Option Swaps [Member]
Derivative [Line Items]
Notional Value	64	11
Asset Derivatives	54	107
Liability Derivatives	7	88
Total Return Swaps [Member]
Derivative [Line Items]
Notional Value	103,916	125,913
Asset Derivatives	1,993	1,388
Liability Derivatives	$ 1,187	$ 3,744