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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (419)	$ (5,387)	$ 1,948	$ (14,879)
Exchange-traded futures
Derivative [Line Items]
Net (losses) gains on derivative instruments	717	(3,301)	1,542	(8,833)
Currency forwards
Derivative [Line Items]
Net (losses) gains on derivative instruments	257	132	274	(930)
Interest rate swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(7)	(151)	267	(248)
Credit default swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(169)	(237)	(95)	(909)
Total return swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (1,217)	$ (1,830)	$ (40)	$ (3,959)