Stockholders' equity - Other (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) D item $ / shares shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) D item $ / shares shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares
Stockholders' equity
Shares available for issuance	2,700		2,700
Valuation assumptions
Risk-free interest rate, minimum (as a percent)			2.70%
Risk-free interest rate, maximum (as a percent)			2.80%
Common Class A
Stockholders' equity
Shares authorized for issuance	7,200		7,200
Stock option and restricted stock
Stock-based compensation
Stock-based compensation expense (in dollars) | $	$ 1,000	$ 1,000	$ 3,000	$ 3,100
Stock options
Stock-based compensation
Unrecognized compensation cost related to unvested stock options (in dollars) | $	$ 1,000		$ 1,000
Weighted-average periods over which unrecognized compensation cost recognized			1 year 1 month 6 days
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)			2,898
Granted (in shares)			49
Exercised (in shares)			(67)
Forfeited (in shares)			(24)
Expired (in shares)			(6)
Outstanding at the end of the period (in shares)	2,850		2,850		2,898
Vested at the end of the period (in shares)	2,143		2,143
Exercisable at the end of the period (in shares)	2,143		2,143
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 11.62
Granted (in dollars per share) | $ / shares			11.42
Exercised (in dollars per share) | $ / shares			13.38
Forfeited (in dollars per share) | $ / shares			12.30
Expired (in dollars per share) | $ / shares			12.10
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 11.57		11.57		$ 11.62
Vested at the end of the period (in dollars per share) | $ / shares	11.70		11.70
Exercisable at the end of the period (in dollars per share) | $ / shares	$ 11.70		$ 11.70
Weighted-average remaining contractual term
Outstanding			5 years 9 months 18 days		6 years 6 months
Granted			6 years
Vested at the end of the period			5 years 4 months 24 days
Exercisable at the end of the period			5 years 4 months 24 days
Aggregate intrinsic value
Outstanding at the beginning of the period (in dollars) | $			$ 1,738
Outstanding at the end of the period (in dollars) | $	$ 7,169		7,169		$ 1,738
Vested at the end of the period (in dollars) | $	5,196		5,196
Exercisable at the end of the period (in dollars) | $	5,196		$ 5,196
Weighted-average grant date fair value of options granted (in dollars per share) | $ / shares			$ 4.80
Restricted Stock
Stock-based compensation
Unrecognized compensation cost related to unvested restricted stock (in dollars) | $	$ 1,700		$ 1,700
Weighted-average periods over which unrecognized compensation cost recognized			1 year
Number of shares
Outstanding at the beginning of the period (in shares)			504
Granted (in shares)			93
Vested (in shares)			(199)
Forfeited (in shares)			(9)
Outstanding at the end of the period (in shares)	389		389		504
Weighted-average grant date fair value
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 10.23
Granted (in dollars per share) | $ / shares			11.85
Vested (in dollars per share) | $ / shares			11.50
Forfeited (in dollars per share) | $ / shares			11.85
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 9.93		$ 9.93		$ 10.23
Time Based Restricted Stock 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 (as a percent)					2.20%
Volatility (as a percent)			39.00%		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 Restricted Stock and Stock Option | Common Class A
Stockholders' equity
Closing price per share to be attained for vesting of awards to begin (in dollars per share) | $ / shares			$ 15.00
Event Based Restricted Stock and Stock Option | Common Class A | 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
Event Based Stock Option
Aggregate intrinsic value
Unvested options	300		300
Event Based Restricted Stock
Aggregate intrinsic value
Unvested options	200		200