Derivative Instruments - Gains and Losses for Derivative Instruments Recognized in Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Derivative [Line Items]
Net gains on derivative instruments	$ 19,732	$ 2,336
Exchange-traded futures
Derivative [Line Items]
Net gains on derivative instruments	7,513	1,135
Currency forwards
Derivative [Line Items]
Net gains on derivative instruments	544	1,181
Interest rate swaps
Derivative [Line Items]
Net gains on derivative instruments	(73)	88
Credit default swaps
Derivative [Line Items]
Net gains on derivative instruments	1,717	(522)
Total return swaps
Derivative [Line Items]
Net gains on derivative instruments	7,406	(907)
Option swaps
Derivative [Line Items]
Net gains on derivative instruments	$ 2,625	$ 1,361