Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Value	$ 620,078	$ 612,029
Asset Derivatives	11,471	17,079
Liability Derivatives	21,329	14,822
Exchange-traded futures
Derivative [Line Items]
Notional Value	185,680	218,657
Asset Derivatives	220	1,594
Liability Derivatives	3,075	2,534
Currency forwards
Derivative [Line Items]
Notional Value	90,925	87,019
Asset Derivatives	7,116	7,647
Liability Derivatives	7,241	7,582
Interest rate swaps
Derivative [Line Items]
Notional Value	95,146	112,658
Asset Derivatives	1,885	1,649
Liability Derivatives	2,537	1,959
Credit default swaps
Derivative [Line Items]
Notional Value	150,244	94,657
Asset Derivatives	2,211	2,888
Liability Derivatives	5,832	2,685
Total return swaps
Derivative [Line Items]
Notional Value	98,083	99,038
Asset Derivatives	39	3,301
Liability Derivatives	$ 2,644	$ 62