Derivative Instruments (Notional Value and Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional Value	$ 709,558	$ 584,535
Derivative Assets	32,566	13,491
Derivative Liabilities	40,056	19,259
Exchange-traded futures
Derivative [Line Items]
Notional Value	175,080	171,112
Derivative Assets	1,505	939
Derivative Liabilities	4,895	871
Currency forwards
Derivative [Line Items]
Notional Value	85,003	60,809
Derivative Assets	8,414	8,545
Derivative Liabilities	7,943	8,633
Interest rate swaps
Derivative [Line Items]
Notional Value	82,780	92,756
Derivative Assets	2,886	1,746
Derivative Liabilities	3,641	2,254
Credit default swaps
Derivative [Line Items]
Notional Value	275,733	168,303
Derivative Assets	16,238	2,151
Derivative Liabilities	15,294	5,611
Total return swaps
Derivative [Line Items]
Notional Value	90,608	91,201
Derivative Assets	2,351	110
Derivative Liabilities	8,283	1,764
Option swaps
Derivative [Line Items]
Notional Value	354	354
Derivative Assets	1,172	0
Derivative Liabilities	$ 0	$ 126