Derivative Instruments - Notional Value and Fair Value of Derivative Instruments (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional Value	$ 647,050	$ 584,535
Derivative Assets	24,224	13,491
Derivative Liabilities	26,626	19,259
Exchange-traded futures
Derivative [Line Items]
Notional Value	173,988	171,112
Derivative Assets	113	939
Derivative Liabilities	1,126	871
Currency forwards
Derivative [Line Items]
Notional Value	87,263	60,809
Derivative Assets	8,339	8,545
Derivative Liabilities	7,873	8,633
Interest rate swaps
Derivative [Line Items]
Notional Value	79,327	92,756
Derivative Assets	2,512	1,746
Derivative Liabilities	3,328	2,254
Credit default swaps
Derivative [Line Items]
Notional Value	217,575	168,303
Derivative Assets	11,957	2,151
Derivative Liabilities	10,793	5,611
Total return swaps
Derivative [Line Items]
Notional Value	86,431	91,201
Derivative Assets	599	110
Derivative Liabilities	2,143	1,764
Option swaps
Derivative [Line Items]
Notional Value	2,466	354
Derivative Assets	704	0
Derivative Liabilities	$ 1,363	$ 126