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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ 792	$ 6,222	$ (23,942)
Net realized and unrealized (losses) gains on investments | Interest rate futures
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(5,594)	(3,252)	(17,379)
Net realized and unrealized (losses) gains on investments | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(84)	436	0
Net realized and unrealized (losses) gains on investments | Credit default swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(3,106)	326	1,965
Net foreign exchange (losses) gains | Foreign currency forward contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	3,840	9,628	(6,937)
Net foreign exchange (losses) gains | Foreign currency forward contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	$ 5,736	$ (916)	$ (1,591)