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				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Total	$ (2)		$ (65)		$ (3)		$ (482)
Interest rate swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Loss on derivative instruments-realized	(59)	[1]	(257)	[1]	(133)	[1]	(669)	[1]
Gain on derivative instruments - unrealized	(58)		(217)		(131)		(364)
Interest rate caps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Loss on derivative instruments-unrealized	$ (1)		$ (25)		$ (1)		$ (177)

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