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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative [Line Items]
Net (losses) on derivative instruments	$ (5,387)	$ (5,052)	$ (14,879)	$ (8,609)
Exchange-traded futures
Derivative [Line Items]
Net (losses) on derivative instruments	(3,301)	(913)	(8,833)	2,433
Currency forwards
Derivative [Line Items]
Net (losses) on derivative instruments	132	(685)	(930)	(1,820)
Interest rate swaps
Derivative [Line Items]
Net (losses) on derivative instruments	(151)	(559)	(248)	(1,931)
Credit default swaps
Derivative [Line Items]
Net (losses) on derivative instruments	(237)	(198)	(909)	(606)
Option swaps
Derivative [Line Items]
Net (losses) on derivative instruments	0	39	0	87
Total return swaps
Derivative [Line Items]
Net (losses) on derivative instruments	$ (1,830)	$ (2,736)	$ (3,959)	$ (6,772)