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		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ (10,325)	$ 6,189	$ (37,006)	$ 14,802
Net realized and unrealized gains (losses) on investments | Interest Rate Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(9,456)	19,784	(28,815)	15,376
Net realized and unrealized gains (losses) on investments | Credit Default Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	305	22	215	62
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	170
Net foreign exchange losses | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(3,815)	(10,210)	(5,189)	(6,599)
Net foreign exchange losses | Foreign Currency Forward Contracts, Investment Operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ 2,641	$ (3,417)	$ (3,217)	$ 5,793