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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (8,042)	$ 23,733	$ (16,651)	$ 20,151
Exchange-traded futures
Derivative [Line Items]
Net (losses) gains on derivative instruments	(4,975)	16,683	(2,542)	13,124
Currency forwards
Derivative [Line Items]
Net (losses) gains on derivative instruments	(641)	5,900	(2,461)	8,058
Interest rate swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(251)	(438)	(2,182)	(768)
Credit default swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(352)	422	(958)	207
Option swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(157)	80	(70)	59
Total return swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (1,666)	$ 1,086	$ (8,438)	$ (529)