Quarterly financial data (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly financial data (unaudited)
Revenue	$ 33,627us-gaap_SalesRevenueNet	$ 30,822us-gaap_SalesRevenueNet	$ 28,396us-gaap_SalesRevenueNet	$ 26,452us-gaap_SalesRevenueNet	$ 28,766us-gaap_SalesRevenueNet	$ 28,607us-gaap_SalesRevenueNet	$ 26,239us-gaap_SalesRevenueNet	$ 23,134us-gaap_SalesRevenueNet	$ 119,297us-gaap_SalesRevenueNet	$ 106,746us-gaap_SalesRevenueNet	$ 102,506us-gaap_SalesRevenueNet
(Loss) income from operations	(6,014)us-gaap_OperatingIncomeLoss	(3,487)us-gaap_OperatingIncomeLoss	(3,352)us-gaap_OperatingIncomeLoss	(5,173)us-gaap_OperatingIncomeLoss	(752)us-gaap_OperatingIncomeLoss	784us-gaap_OperatingIncomeLoss	(424)us-gaap_OperatingIncomeLoss	(1,502)us-gaap_OperatingIncomeLoss	(18,026)us-gaap_OperatingIncomeLoss	(1,894)us-gaap_OperatingIncomeLoss	10,846us-gaap_OperatingIncomeLoss
Net (loss) income attributable to common stockholders	$ (6,524)	$ (3,815)	$ (3,734)	$ (5,448)	$ (2,802)	$ 354	$ (399)	$ (1,121)	$ (19,521)	$ (3,968)	$ 7,295
Basic (loss) earnings per share (in dollars per share)	$ (0.18)us-gaap_EarningsPerShareBasic	$ (0.11)us-gaap_EarningsPerShareBasic	$ (0.10)us-gaap_EarningsPerShareBasic	$ (0.15)us-gaap_EarningsPerShareBasic	$ (0.08)us-gaap_EarningsPerShareBasic	$ 0.01us-gaap_EarningsPerShareBasic	$ (0.01)us-gaap_EarningsPerShareBasic	$ (0.03)us-gaap_EarningsPerShareBasic	$ (0.55)us-gaap_EarningsPerShareBasic	$ (0.11)us-gaap_EarningsPerShareBasic	$ 0.21us-gaap_EarningsPerShareBasic
Diluted (loss) earnings per share (in dollars per share)	$ (0.18)us-gaap_EarningsPerShareDiluted	$ (0.11)us-gaap_EarningsPerShareDiluted	$ (0.10)us-gaap_EarningsPerShareDiluted	$ (0.15)us-gaap_EarningsPerShareDiluted	$ (0.08)us-gaap_EarningsPerShareDiluted	$ 0.01us-gaap_EarningsPerShareDiluted	$ (0.01)us-gaap_EarningsPerShareDiluted	$ (0.03)us-gaap_EarningsPerShareDiluted	$ (0.55)us-gaap_EarningsPerShareDiluted	$ (0.11)us-gaap_EarningsPerShareDiluted	$ 0.20us-gaap_EarningsPerShareDiluted

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (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.

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

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

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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.

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