Derivative Instruments (Gains and Losses for Derivative Instruments Recognized in Investment Gains (Losses)) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (9,117)	$ (419)	$ (28,882)	$ 1,948
Exchange-traded futures
Derivative [Line Items]
Net (losses) gains on derivative instruments	(3,436)	717	(8,551)	1,542
Currency forwards
Derivative [Line Items]
Net (losses) gains on derivative instruments	(75)	257	(115)	274
Interest rate swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(331)	(7)	(645)	267
Credit default swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(1,465)	(169)	(3,805)	(95)
Total return swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (3,810)	$ (1,217)	$ (15,766)	$ (40)