Risk Management and Use of Derivative Financial Instruments - Derivative Gain Loss Recognized in Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	$ 230	$ (149)
Derivatives Not Designated as Hedging Instruments | Stock warrants | Other gains and (losses)
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	132	(198)
Derivatives Not Designated as Hedging Instruments | Foreign currency forward contracts | Other gains and (losses)
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	66	33
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Interest expense
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	(8)	18
Derivatives Not Designated as Hedging Instruments | Swaption | Other gains and (losses)
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	0	(48)
Derivatives in Cash Flow Hedging Relationships | Interest rate swaps | Interest expense
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	45	46
Derivatives in Cash Flow Hedging Relationships | Foreign currency collars | Other gains and (losses)
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	$ (5)	$ 0