Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Value	$ 363,470	$ 459,743
Asset Derivatives	5,846	8,413
Liability Derivatives	6,600	8,649
Exchange-traded futures
Derivative [Line Items]
Notional Value	88,695	103,108
Asset Derivatives	1,111	1,224
Liability Derivatives	962	1,092
Currency forwards
Derivative [Line Items]
Notional Value	139,983	180,820
Asset Derivatives	3,077	4,541
Liability Derivatives	3,119	4,711
Interest rate swaps
Derivative [Line Items]
Notional Value	32,526	40,664
Asset Derivatives	951	940
Liability Derivatives	877	897
Credit default swaps
Derivative [Line Items]
Notional Value	38,609	45,108
Asset Derivatives	600	1,205
Liability Derivatives	1,380	905
Total return swaps
Derivative [Line Items]
Notional Value	63,657	90,043
Asset Derivatives	107	503
Liability Derivatives	$ 262	$ 1,044