Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Notional Value	$ 628,788	$ 508,405
Asset Derivatives	18,739	6,787
Liability Derivatives	11,354	9,392
Exchange-traded futures
Derivative [Line Items]
Notional Value	140,160	149,863
Asset Derivatives	2,740	571
Liability Derivatives	264	2,438
Currency forwards
Derivative [Line Items]
Notional Value	296,794	149,282
Asset Derivatives	6,092	1,782
Liability Derivatives	3,644	333
Interest rate swaps
Derivative [Line Items]
Notional Value	73,565	50,591
Asset Derivatives	5,251	1,507
Liability Derivatives	6,666	2,679
Credit default swaps
Derivative [Line Items]
Notional Value	29,092	32,745
Asset Derivatives	2,322	1,432
Liability Derivatives	334	110
Option swaps
Derivative [Line Items]
Notional Value	213	11
Asset Derivatives	81	107
Liability Derivatives	25	88
Total return swaps
Derivative [Line Items]
Notional Value	88,964	125,913
Asset Derivatives	2,253	1,388
Liability Derivatives	$ 421	$ 3,744