Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Value	$ 600,709	$ 475,976
Asset Derivatives	13,565	11,619
Liability Derivatives	13,106	14,323
Exchange-traded futures
Derivative [Line Items]
Notional Value	178,462	163,458
Asset Derivatives	1,517	948
Liability Derivatives	1,240	2,540
Currency forwards
Derivative [Line Items]
Notional Value	157,448	126,503
Asset Derivatives	8,286	8,306
Liability Derivatives	8,347	8,058
Interest rate swaps
Derivative [Line Items]
Notional Value	50,403	43,309
Asset Derivatives	843	951
Liability Derivatives	659	870
Credit default swaps
Derivative [Line Items]
Notional Value	78,741	74,600
Asset Derivatives	1,403	1,247
Liability Derivatives	2,410	2,465
Total return swaps
Derivative [Line Items]
Notional Value	135,655	68,106
Asset Derivatives	1,516	167
Liability Derivatives	$ 450	$ 390