Derivative Instruments (Gains and Losses for Derivative Instruments Recognized in Investment Gains (Losses)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Derivative [Line Items]
Net (losses) on derivative instruments	$ (3,557)	$ (6,055)
Exchange-traded futures
Derivative [Line Items]
Net (losses) on derivative instruments	3,346	(4,987)
Currency forwards
Derivative [Line Items]
Net (losses) on derivative instruments	(1,135)	3,452
Interest rate swaps
Derivative [Line Items]
Net (losses) on derivative instruments	(1,372)	(745)
Credit default swaps
Derivative [Line Items]
Net (losses) on derivative instruments	(408)	(57)
Option swaps
Derivative [Line Items]
Net (losses) on derivative instruments	48	(17)
Total return swaps
Derivative [Line Items]
Net (losses) on derivative instruments	$ (4,036)	$ (3,701)