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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 1,895	$ (14,090)	$ (7,183)	$ (15,437)
Exchange-traded futures
Derivative [Line Items]
Net gains (losses) on derivative instruments	2,186	(2,753)	(1,561)	(7,565)
Currency forwards
Derivative [Line Items]
Net gains (losses) on derivative instruments	512	(1,031)	1,339	(585)
Interest rate swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	(179)	15	(202)	(372)
Credit default swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	(91)	(2,298)	(2,327)	5,492
Total return swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	(611)	(6,840)	(4,498)	(10,691)
Option swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 78	$ (1,183)	$ 66	$ (1,716)