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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (5,052)	$ 2,473	$ (8,609)	$ (3,582)
Exchange-traded futures
Derivative [Line Items]
Net (losses) gains on derivative instruments	(913)	1,428	2,433	(3,559)
Currency forwards
Derivative [Line Items]
Net (losses) gains on derivative instruments	(685)	(1,294)	(1,820)	2,158
Interest rate swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(559)	415	(1,931)	(330)
Credit default swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(198)	(158)	(606)	(215)
Option swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	39	(4)	87	(21)
Total return swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (2,736)	$ 2,086	$ (6,772)	$ (1,615)