Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional Value	$ 386,048	$ 664,558
Asset Derivatives	7,007	12,588
Liability Derivatives	9,548	12,221
Exchange-traded futures
Derivative [Line Items]
Notional Value	82,680	160,755
Asset Derivatives	827	1,539
Liability Derivatives	1,750	2,651
Currency forwards
Derivative [Line Items]
Notional Value	184,612	262,873
Asset Derivatives	3,848	4,604
Liability Derivatives	4,607	4,077
Interest rate swaps
Derivative [Line Items]
Notional Value	36,548	65,484
Asset Derivatives	565	2,945
Liability Derivatives	1,780	3,745
Credit default swaps
Derivative [Line Items]
Notional Value	20,636	29,421
Asset Derivatives	1,153	2,089
Liability Derivatives	385	774
Option swaps
Derivative [Line Items]
Notional Value	157	24
Asset Derivatives	191	9
Liability Derivatives	34	2
Total return swaps
Derivative [Line Items]
Notional Value	61,415	146,001
Asset Derivatives	423	1,402
Liability Derivatives	$ 992	$ 972