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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 2,473	$ (13,945)	$ (3,582)	$ (19,466)
Exchange-Traded Futures [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	1,428	(3,898)	(3,559)	(4,396)
Currency Forwards [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	(1,294)	(1,346)	2,158	(3,012)
Interest Rate Swaps [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	415	(1,134)	(330)	(1,927)
Credit Default Swaps [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	(158)	(374)	(215)	(379)
Option Swaps [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	(4)	(107)	(21)	(142)
Total Return Swaps [Member]
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 2,086	$ (7,086)	$ (1,615)	$ (9,610)