Nature of business (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Numerator for (loss) earnings per common share calculation:
Net (loss) income	$ (7,559)	$ 2,745
Denominator for (loss) earnings per common share calculation:
Weighted-average common shares, basic	38,955	40,514
Effect of dilutive securities:
Stock options, restricted stock and warrants		246
Weighted-average common shares, diluted	38,955	40,760
(Loss Per Share) Earnings Per Share
Basic (in dollars per share)	$ (0.19)	$ 0.07
Diluted (in dollars per share)	$ (0.19)	$ 0.07
Shares excluded from the computation of diluted income per common share	2,100	2,100
Outstanding awards excluded from computation of diluted earnings per share	200
Aggregate purchase price of Class A common stock	$ 34
Common Class A
(Loss Per Share) Earnings Per Share
Par value of common stock (in dollars per share)	$ 0.0001		$ 0.0001
Common Class B
(Loss Per Share) Earnings Per Share
Par value of common stock (in dollars per share)	$ 0.0001		$ 0.0001