Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Value	$ 419,437	$ 459,743
Asset Derivatives	7,876	8,413
Liability Derivatives	8,176	8,649
Exchange-traded futures
Derivative [Line Items]
Notional Value	98,938	103,108
Asset Derivatives	813	1,224
Liability Derivatives	916	1,092
Currency forwards
Derivative [Line Items]
Notional Value	190,292	180,820
Asset Derivatives	5,666	4,541
Liability Derivatives	5,156	4,711
Interest rate swaps
Derivative [Line Items]
Notional Value	34,038	40,664
Asset Derivatives	898	940
Liability Derivatives	970	897
Credit default swaps
Derivative [Line Items]
Notional Value	32,399	45,108
Asset Derivatives	423	1,205
Liability Derivatives	1,015	905
Total return swaps
Derivative [Line Items]
Notional Value	63,770	90,043
Asset Derivatives	76	503
Liability Derivatives	$ 119	$ 1,044