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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (115)	$ (5,755)	$ (28,997)	$ (3,807)
Exchange-traded futures
Derivative [Line Items]
Net (losses) gains on derivative instruments	(1,352)	157	(9,903)	1,699
Currency forwards
Derivative [Line Items]
Net (losses) gains on derivative instruments	1,453	673	1,338	947
Interest rate swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(81)	157	(726)	424
Credit default swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(449)	(1,117)	(4,254)	(1,212)
Total return swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ 314	$ (5,625)	$ (15,452)	$ (5,665)