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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ 6,335	$ (13,610)	$ (30,671)	$ 1,192
Net realized and unrealized gains (losses) on investments | Interest Rate Futures
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	1,040	(16,817)	(27,775)	(1,441)
Net realized and unrealized gains (losses) on investments | Credit Default Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	687	195	902	257
Net realized and unrealized gains (losses) on investments | Weather Contract
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	0	10	0	180
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	5,097	817	(92)	(5,782)
Net foreign exchange (losses) gains | Foreign Currency Forward Contracts, Investment Operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ (489)	$ 2,185	$ (3,706)	$ 7,978