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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative [Line Items]
Net (losses) on derivative instruments	$ (4,891)	$ (8,042)	$ (19,770)	$ (16,651)
Exchange-traded futures
Derivative [Line Items]
Net (losses) on derivative instruments	(3,290)	(4,975)	(12,123)	(2,542)
Currency forwards
Derivative [Line Items]
Net (losses) on derivative instruments	(62)	(641)	(992)	(2,461)
Interest rate swaps
Derivative [Line Items]
Net (losses) on derivative instruments	151	(251)	(97)	(2,182)
Credit default swaps
Derivative [Line Items]
Net (losses) on derivative instruments	(273)	(352)	(1,182)	(958)
Option swaps
Derivative [Line Items]
Net (losses) on derivative instruments	0	(157)	0	(70)
Total return swaps
Derivative [Line Items]
Net (losses) on derivative instruments	$ (1,417)	$ (1,666)	$ (5,376)	$ (8,438)