Risk Management and Use of Derivative Financial Instruments - Derivative Gain Loss Recognized in Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	$ (123)	$ 490	$ 193
Not designated | Interest expense | Interest rate swap
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	11	6	0
Not designated | Other income and (expense) | Swaption
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	(220)	(45)	(196)
Not designated | Other income and (expense) | Stock warrants
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	(33)	66	(66)
Not designated | Other income and (expense) | Foreign currency forward contracts
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	10	0	(16)
Not designated | Other income and (expense) | Embedded derivatives
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	0	0	177
Not designated | Other income and (expense) | Foreign currency collars
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	0	0	(8)
Designated as hedging | Interest expense | Interest rate swap
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	121	463	302
Designated as hedging | Other income and (expense) | Foreign currency collars
Amount of Gain (Loss) Recognized in Income on Derivatives
Amount of gain (loss) recognized income on derivatives	$ (12)	$ 0	$ 0