Stockholders' equity - Other (Details) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) item $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Stockholders' equity
Shares available for issuance	2,700
Stock option and restricted stock
Stock-based compensation
Stock-based compensation expense (in dollars) | $	$ 3,900	$ 4,100
Stock options
Stock-based compensation
Unrecognized compensation cost related to unvested stock options (in dollars) | $	$ 900
Weighted-average periods over which unrecognized compensation cost recognized	1 year 9 months 18 days
Estimated forfeitures (as a percent)	1.50%
Valuation assumptions
Dividend yield (as a percent)	0.00%
Number of shares
Outstanding at the beginning of the period (in shares)	2,898	2,920
Granted (in shares)	109	55
Exercised (in shares)	(67)
Forfeited (in shares)	(24)	(37)
Expired (in shares)	(6)	(40)
Outstanding at the end of the period (in shares)	2,910	2,898	2,920
Vested at the end of the period (in shares)	2,175
Exercisable at the end of the period (in shares)	2,175
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 11.62	$ 11.64
Granted (in dollars per share) | $ / shares	12.84	11.35
Exercised (in dollars per share) | $ / shares	13.38
Forfeited (in dollars per share) | $ / shares	12.30	10.39
Expired (in dollars per share) | $ / shares	12.10	11.51
Outstanding at the end of the period (in dollars per share) | $ / shares	11.62	$ 11.62	$ 11.64
Vested at the end of the period (in dollars per share) | $ / shares	11.71
Exercisable at the end of the period (in dollars per share) | $ / shares	$ 11.71
Weighted-average remaining contractual term
Outstanding	5 years 7 months 6 days	6 years 6 months	7 years 7 months 6 days
Granted	6 years	6 years
Vested at the end of the period	5 years 3 months 18 days
Exercisable at the end of the period	5 years 3 months 18 days
Aggregate intrinsic value
Outstanding at the beginning of the period (in dollars) | $	$ 1,738	$ 1,274
Outstanding at the end of the period (in dollars) | $	2,806	$ 1,738	$ 1,274
Vested at the end of the period (in dollars) | $	2,102
Exercisable at the end of the period (in dollars) | $	$ 2,102
Weighted-average grant date fair value of options granted (in dollars per share) | $ / shares	$ 5.49	$ 3.39
Restricted Stock
Stock-based compensation
Unrecognized compensation cost related to unvested restricted stock (in dollars) | $	$ 1,100
Weighted-average periods over which unrecognized compensation cost recognized	10 months 24 days
Number of shares
Outstanding at the beginning of the period (in shares)	504	561
Granted (in shares)	93	154
Vested (in shares)	(218)	(203)
Forfeited (in shares)	(9)	(8)
Outstanding at the end of the period (in shares)	370	504	561
Weighted-average grant date fair value
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 10.23	$ 10.58
Granted (in dollars per share) | $ / shares	11.85	11.19
Vested (in dollars per share) | $ / shares	11.49	11.80
Forfeited (in dollars per share) | $ / shares	11.85	13.38
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 9.86	$ 10.23	$ 10.58
Time Based Restricted Stock Awards and Stock Option
Stockholders' equity
Number of equal annual installments for vesting of awards | item	3
Time Based Stock Option | Black Scholes Pricing Model
Valuation assumptions
Risk-free interest rate, minimum (as a percent)	2.70%
Risk-free interest rate, maximum (as a percent)	3.00%
Risk-free interest rate (as a percent)		2.20%
Volatility, minimum (as a percent)	39.00%
Volatility, maximum (as a percent)	41.00%
Volatility (as a percent)		25.80%
Weighted-average expected term(years)	6 years	6 years
Time Based Restricted Stock
Weighted-average grant date fair value
Vesting period	3 years
Event Based Stock Option
Aggregate intrinsic value
Unvested options	300
Event Based Restricted Stock
Aggregate intrinsic value
Unvested options	200
Common Class A
Stockholders' equity
Shares authorized for issuance	7,200
Common Class A | Event Based Restricted Stock and Stock Option
Stockholders' equity
Closing price per share to be attained for vesting of awards to begin (in dollars per share) | $ / shares	$ 15.00
Common Class A | Event Based Restricted Stock and Stock Option | Minimum
Stockholders' equity
Number of trading days on which the closing price per share should attain the specified price per share for vesting of awards to begin | item	10