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
	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ 371	$ (26,681)
Net realized and unrealized gains on investments | Interest Rate Futures
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(158)	(19,359)
Net realized and unrealized gains on investments | Credit Default Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	102	(90)
Net foreign exchange gains (losses) | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	1,060	(1,374)
Net foreign exchange gains (losses) | Foreign Currency Forward Contracts, Investment Operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ (633)	$ (5,858)