Derivative Instruments (Gains and Losses for Derivative Instruments Recognized in Investment Gains (Losses)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative [Line Items]
Net (losses) on derivative instruments	$ (9,492)	$ (3,557)
Exchange-traded futures
Derivative [Line Items]
Net (losses) on derivative instruments	(5,532)	3,346
Currency forwards
Derivative [Line Items]
Net (losses) on derivative instruments	(1,062)	(1,135)
Interest rate swaps
Derivative [Line Items]
Net (losses) on derivative instruments	(97)	(1,372)
Credit default swaps
Derivative [Line Items]
Net (losses) on derivative instruments	(672)	(408)
Option swaps
Derivative [Line Items]
Net (losses) on derivative instruments	0	48
Total return swaps
Derivative [Line Items]
Net (losses) on derivative instruments	$ (2,129)	$ (4,036)