Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Value	$ 515,414	$ 612,029
Asset Derivatives	10,054	17,079
Liability Derivatives	14,390	14,822
Exchange-traded futures
Derivative [Line Items]
Notional Value	168,743	218,657
Asset Derivatives	231	1,594
Liability Derivatives	1,672	2,534
Currency forwards
Derivative [Line Items]
Notional Value	57,539	87,019
Asset Derivatives	7,044	7,647
Liability Derivatives	6,960	7,582
Interest rate swaps
Derivative [Line Items]
Notional Value	99,922	112,658
Asset Derivatives	1,391	1,649
Liability Derivatives	1,815	1,959
Credit default swaps
Derivative [Line Items]
Notional Value	96,504	94,657
Asset Derivatives	1,347	2,888
Liability Derivatives	3,481	2,685
Total return swaps
Derivative [Line Items]
Notional Value	92,706	99,038
Asset Derivatives	41	3,301
Liability Derivatives	$ 462	$ 62