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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 23,733	$ 4,420	$ 20,151	$ (15,046)
Exchange-traded futures
Derivative [Line Items]
Net gains (losses) on derivative instruments	16,683	1,715	13,124	(2,681)
Currency forwards
Derivative [Line Items]
Net gains (losses) on derivative instruments	5,900	3,462	8,058	450
Interest rate swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	(438)	170	(768)	(1,757)
Credit default swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	422	320	207	(59)
Option swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	80	(93)	59	(235)
Total return swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 1,086	$ (1,154)	$ (529)	$ (10,764)