Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional Value	$ 402,798	$ 664,558
Asset Derivatives	6,944	12,588
Liability Derivatives	7,050	12,221
Exchange-traded futures
Derivative [Line Items]
Notional Value	91,229	160,755
Asset Derivatives	1,197	1,539
Liability Derivatives	600	2,651
Currency forwards
Derivative [Line Items]
Notional Value	137,224	262,873
Asset Derivatives	3,275	4,604
Liability Derivatives	3,437	4,077
Interest rate swaps
Derivative [Line Items]
Notional Value	42,608	65,484
Asset Derivatives	798	2,945
Liability Derivatives	1,664	3,745
Credit default swaps
Derivative [Line Items]
Notional Value	46,165	29,421
Asset Derivatives	1,421	2,089
Liability Derivatives	903	774
Option swaps
Derivative [Line Items]
Notional Value		24
Asset Derivatives		9
Liability Derivatives		2
Total return swaps
Derivative [Line Items]
Notional Value	85,572	146,001
Asset Derivatives	253	1,402
Liability Derivatives	$ 446	$ 972