Derivative Instruments (Gains and Losses for Derivative Instruments Recognized in Investment Gains (Losses)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (19,765)	$ 2,367
Exchange-traded futures
Derivative [Line Items]
Net (losses) gains on derivative instruments	(5,115)	825
Currency forwards
Derivative [Line Items]
Net (losses) gains on derivative instruments	(40)	17
Interest rate swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(314)	274
Credit default swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	(2,340)	74
Total return swaps
Derivative [Line Items]
Net (losses) gains on derivative instruments	$ (11,956)	$ 1,177