Nature of business (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) segment $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Dec. 31, 2015 $ / shares
Nature of business
Number of operating segments | segment			1
Numerator for earnings per common share calculation:
Net income | $	$ 4,349	$ 2,910	$ 12,078	$ 8,805
Denominator for earnings per common share calculation:
Weighted-average common shares, basic | shares	40,499	43,103	42,057	42,748
Effect of dilutive securities:
Stock options, restricted stock and warrants | shares	536	1,040	707	815
Weighted-average common shares, diluted | shares	41,035	44,143	42,764	43,563
EPS
Basic (in dollars per share) | $ / shares	$ 0.11	$ 0.07	$ 0.29	$ 0.21
Diluted (in dollars per share) | $ / shares	$ 0.11	$ 0.07	$ 0.28	$ 0.20
Shares excluded from the computation of diluted income (loss) per common share | shares	1,000		1,000
Common Class A
EPS
Par value of common stock (in dollars per share) | $ / shares	$ 0.0001		$ 0.0001		$ 0.0001
Common Class B
EPS
Par value of common stock (in dollars per share) | $ / shares	$ 0.0001		$ 0.0001		$ 0.0001