Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Value	$ 541,125	$ 475,976
Asset Derivatives	14,113	11,619
Liability Derivatives	14,552	14,323
Exchange-traded futures
Derivative [Line Items]
Notional Value	155,213	163,458
Asset Derivatives	2,105	948
Liability Derivatives	985	2,540
Currency forwards
Derivative [Line Items]
Notional Value	105,475	126,503
Asset Derivatives	8,558	8,306
Liability Derivatives	9,044	8,058
Interest rate swaps
Derivative [Line Items]
Notional Value	72,056	43,309
Asset Derivatives	1,125	951
Liability Derivatives	1,015	870
Credit default swaps
Derivative [Line Items]
Notional Value	81,760	74,600
Asset Derivatives	1,639	1,247
Liability Derivatives	2,562	2,465
Total return swaps
Derivative [Line Items]
Notional Value	126,621	68,106
Asset Derivatives	686	167
Liability Derivatives	$ 946	$ 390