Derivative instruments - Summary of Amounts Recognized on Consolidated Statements of Operations Related to Company's Derivatives (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Total	$ 1		$ (87)		$ (1)		$ (569)
Interest rate swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Loss on derivative instruments-realized	(24)	[1]	(127)	[1]	(156)	[1]	(796)	[1]
Gain on derivative instruments - unrealized	25		50		155		414
Interest rate caps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Loss on derivative instruments-unrealized			$ (10)				$ (187)

[1] Realized losses represent net amounts accrued for the Company’s derivative instruments during the period.