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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (5,755)	$ (4,891)	$ (3,807)	$ (19,770)
Exchange-traded futures
Derivative [Line Items]
Net (losses) gains on derivative instruments	157	(3,290)	1,699	(12,123)
Currency forwards
Derivative [Line Items]
Net (losses) gains on derivative instruments	673	(62)	947	(992)
Interest rate swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	157	151	424	(97)
Credit default swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(1,117)	(273)	(1,212)	(1,182)
Total return swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (5,625)	$ (1,417)	$ (5,665)	$ (5,376)