Derivative Instruments (Gain (Loss) Recognized In The Consolidated Statements Of Operations Related To Its Derivative Instruments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ (23,942)	$ 12,362	$ (13,851)
Net realized and unrealized gains (losses) on investments | Interest Rate Futures
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives			(32,713)
Net realized and unrealized gains (losses) on investments | Interest Rate Futures | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(17,379)	5,573
Net realized and unrealized gains (losses) on investments | Credit Default Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives			328
Net realized and unrealized gains (losses) on investments | Credit Default Swaps | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	1,965	(313)
Net realized and unrealized gains (losses) on investments | Weather Contract
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives			1,454
Net realized and unrealized gains (losses) on investments | Weather Contract | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	0	183
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			4,457
Net foreign exchange (losses) gains | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(6,937)	(1,943)
Net foreign exchange (losses) gains | Foreign Currency Forward Contracts, Investment Operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives			$ 12,623
Net foreign exchange (losses) gains | Foreign Currency Forward Contracts, Investment Operations | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ (1,591)	$ 8,862