Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Asset Derivatives	$ 14,710	$ 17,079
Liability Derivatives	16,658	14,822
Derivative, Notional Amount	663,238	612,029
Exchange-traded futures
Derivative [Line Items]
Asset Derivatives	1,282	1,594
Liability Derivatives	240	2,534
Derivative, Notional Amount	184,418	218,657
Currency forwards
Derivative [Line Items]
Asset Derivatives	8,388	7,647
Liability Derivatives	7,716	7,582
Derivative, Notional Amount	64,489	87,019
Interest rate swaps
Derivative [Line Items]
Asset Derivatives	2,395	1,649
Liability Derivatives	3,046	1,959
Derivative, Notional Amount	93,192	112,658
Credit default swaps
Derivative [Line Items]
Asset Derivatives	2,056	2,888
Liability Derivatives	5,567	2,685
Derivative, Notional Amount	232,116	94,657
Total return swaps
Derivative [Line Items]
Asset Derivatives	589	3,301
Liability Derivatives	89	62
Derivative, Notional Amount	$ 89,023	$ 99,038