Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Value	$ 480,256	$ 459,743
Asset Derivatives	7,768	8,413
Liability Derivatives	10,347	8,649
Exchange-traded futures
Derivative [Line Items]
Notional Value	93,712	103,108
Asset Derivatives	536	1,224
Liability Derivatives	1,909	1,092
Currency forwards
Derivative [Line Items]
Notional Value	196,853	180,820
Asset Derivatives	5,742	4,541
Liability Derivatives	5,690	4,711
Interest rate swaps
Derivative [Line Items]
Notional Value	54,105	40,664
Asset Derivatives	635	940
Liability Derivatives	708	897
Credit default swaps
Derivative [Line Items]
Notional Value	39,988	45,108
Asset Derivatives	687	1,205
Liability Derivatives	1,157	905
Total return swaps
Derivative [Line Items]
Notional Value	95,598	90,043
Asset Derivatives	168	503
Liability Derivatives	$ 883	$ 1,044