Derivative Instruments (Gains and Losses for Derivative Instruments Recognized in Investment Gains (Losses)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 29,566	$ (19,765)
Exchange-traded futures
Derivative [Line Items]
Net gains (losses) on derivative instruments	1,006	(5,115)
Currency forwards
Derivative [Line Items]
Net gains (losses) on derivative instruments	658	(40)
Interest rate swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	(612)	(314)
Credit default swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	12,101	(2,340)
Total return swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	15,115	(11,956)
Option swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 1,298	$ 0