Derivative Instruments - Notional Value and Fair Value of Derivative Instruments (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional Value	$ 607,683	$ 584,535
Derivative Assets	26,670	13,491
Derivative Liabilities	28,230	19,259
Exchange-traded futures
Derivative [Line Items]
Notional Value	131,895	171,112
Derivative Assets	317	939
Derivative Liabilities	223	871
Currency forwards
Derivative [Line Items]
Notional Value	68,643	60,809
Derivative Assets	8,271	8,545
Derivative Liabilities	7,709	8,633
Interest rate swaps
Derivative [Line Items]
Notional Value	71,469	92,756
Derivative Assets	2,226	1,746
Derivative Liabilities	3,067	2,254
Credit default swaps
Derivative [Line Items]
Notional Value	282,347	168,303
Derivative Assets	14,770	2,151
Derivative Liabilities	14,075	5,611
Total return swaps
Derivative [Line Items]
Notional Value	50,863	91,201
Derivative Assets	427	110
Derivative Liabilities	656	1,764
Option swaps
Derivative [Line Items]
Notional Value	2,466	354
Derivative Assets	659	0
Derivative Liabilities	$ 2,500	$ 126