Derivative Instruments (Gain (Loss) Recognized In The Consolidated Statements Of Operations Related To Its Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ (2,004)	$ 6,335	$ 3,055	$ (30,671)
Net realized and unrealized gains on investments | Interest rate futures
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(3,016)	1,040	(4,064)	(27,775)
Net realized and unrealized gains on investments | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	271	0	271	0
Net realized and unrealized gains on investments | Credit default swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(1,275)	687	(551)	902
Net foreign exchange (losses) gains | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	2,056	5,097	8,339	(92)
Net foreign exchange (losses) gains | Foreign Currency Forward Contracts, Investment Operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ (40)	$ (489)	$ (940)	$ (3,706)