Derivative Instruments (Gains and Losses for Derivative Instruments Recognized in Investment Gains (Losses)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 2,367	$ (9,492)
Exchange-traded futures
Derivative [Line Items]
Net gains (losses) on derivative instruments	825	(5,532)
Currency forwards
Derivative [Line Items]
Net gains (losses) on derivative instruments	17	(1,062)
Interest rate swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	274	(97)
Credit default swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	74	(672)
Total return swaps
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 1,177	$ (2,129)