Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional Value	$ 450,731	$ 664,558
Asset Derivatives	9,154	12,588
Liability Derivatives	15,385	12,221
Exchange-traded futures
Derivative [Line Items]
Notional Value	76,382	160,755
Asset Derivatives	373	1,539
Liability Derivatives	2,957	2,651
Currency forwards
Derivative [Line Items]
Notional Value	198,361	262,873
Asset Derivatives	4,658	4,604
Liability Derivatives	5,972	4,077
Interest rate swaps
Derivative [Line Items]
Notional Value	43,300	65,484
Asset Derivatives	2,144	2,945
Liability Derivatives	3,270	3,745
Credit default swaps
Derivative [Line Items]
Notional Value	23,004	29,421
Asset Derivatives	1,476	2,089
Liability Derivatives	345	774
Option swaps
Derivative [Line Items]
Notional Value	43	24
Asset Derivatives	46	9
Liability Derivatives	53	2
Total return swaps
Derivative [Line Items]
Notional Value	109,641	146,001
Asset Derivatives	457	1,402
Liability Derivatives	$ 2,788	$ 972