Derivative Instruments - Gains and Losses for Derivative Instruments Recognized in Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 8,936	$ 14,539	$ (2,419)	$ 56,370
Exchange-traded futures
Derivative [Line Items]
Net gains (losses) on derivative instruments	4,622	9,388	2,022	23,342
Currency forwards
Derivative [Line Items]
Net gains (losses) on derivative instruments	594	1,318	557	3,600
Interest rate swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	93	45	144	(7)
Credit default swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	(272)	(1,079)	(4,513)	5,358
Total return swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	2,858	2,970	(843)	18,401
Option swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 1,041	$ 1,897	$ 214	$ 5,676