Stockholders' equity - Other (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) D item $ / shares shares	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) D item $ / shares shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares
Stockholders' equity
Shares available for issuance	2,600		2,600
Stock option and restricted stock
Stock-based compensation
Stock-based compensation expense (in dollars) | $	$ 400	$ 1,000	$ 1,400	$ 2,000
Stock options
Stock-based compensation
Unrecognized compensation cost related to unvested stock options (in dollars) | $	$ 500		$ 500
Weighted-average periods over which unrecognized compensation cost recognized			2 years
Estimated forfeitures (as a percent)	1.50%		1.50%
Valuation assumptions
Dividend yield (as a percent)			0.00%
Number of shares
Outstanding at the beginning of the period (in shares)			2,910
Exercised (in shares)			(9)
Outstanding at the end of the period (in shares)	2,901		2,901		2,910
Vested at the end of the period (in shares)	2,464		2,464
Exercisable at the end of the period (in shares)	2,464		2,464
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 11.62
Exercised (in dollars per share) | $ / shares			11.63
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 11.62		11.62		$ 11.62
Vested at the end of the period (in dollars per share) | $ / shares	11.73		11.73
Exercisable at the end of the period (in dollars per share) | $ / shares	$ 11.73		$ 11.73
Weighted-average remaining contractual term
Outstanding			5 years 1 month 6 days		5 years 7 months 6 days
Vested at the end of the period			5 years 1 month 6 days
Exercisable at the end of the period			5 years 1 month 6 days
Aggregate intrinsic value
Outstanding at the beginning of the period (in dollars) | $			$ 2,806
Outstanding at the end of the period (in dollars) | $	$ 4,649		4,649		$ 2,806
Vested at the end of the period (in dollars) | $	3,725		3,725
Exercisable at the end of the period (in dollars) | $	3,725		3,725
Restricted Stock
Stock-based compensation
Unrecognized compensation cost related to unvested restricted stock (in dollars) | $	$ 1,300		$ 1,300
Weighted-average periods over which unrecognized compensation cost recognized			1 year
Number of shares
Outstanding at the beginning of the period (in shares)			370
Granted (in shares)			76
Vested (in shares)			(184)
Outstanding at the end of the period (in shares)	262		262		370
Weighted-average grant date fair value
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 9.86
Granted (in dollars per share) | $ / shares			14.46
Vested (in dollars per share) | $ / shares			11.88
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 9.76		$ 9.76		$ 9.86
Time Based Restricted Stock Awards and Stock Option
Stockholders' equity
Number of equal annual installments for vesting of awards | item	3		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%
Volatility, minimum (as a percent)					39.00%
Volatility, maximum (as a percent)					41.00%
Weighted-average expected term (years)					6 years
Time Based Restricted Stock
Aggregate intrinsic value
Vesting period			3 years
Event Based Stock Option
Aggregate intrinsic value
Unvested options	300		300
Event Based Restricted Stock
Aggregate intrinsic value
Unvested options	200		200
Common Class A
Stockholders' equity
Shares authorized for issuance	7,200		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 | D	10		10