Quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly financial data (unaudited)
Revenue	$ 38,771	$ 37,186	$ 34,277	$ 29,392	$ 33,627	$ 30,822	$ 28,396	$ 26,452	$ 139,626	$ 119,297	$ 106,746
Loss from operations	(3,504)	(4,759)	(5,765)	(7,603)	(6,014)	(3,487)	(3,352)	(5,173)	(21,631)	(18,026)	(1,894)
Net loss attributable to common stockholders	$ (3,654)	$ (4,819)	$ (5,937)	$ (7,882)	$ (6,524)	$ (3,815)	$ (3,734)	$ (5,448)	$ (22,292)	$ (19,521)	$ (3,968)
Basic and diluted loss per share (in dollars per share)	$ (0.10)	$ (0.13)	$ (0.16)	$ (0.22)	$ (0.18)	$ (0.11)	$ (0.10)	$ (0.15)	$ (0.60)	$ (0.55)	$ (0.11)

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

No definition available.

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- Definition

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

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- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

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- References

No definition available.

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- Definition

Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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