Stockholders' equity - (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) item $ / shares shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) item $ / shares shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) $ / shares shares	May 16, 2016 shares
Stock-based compensation
Stock-based compensation expense (in dollars) | $	$ 1,000	$ 500	$ 3,100	$ 2,500
Common Class A
Equity incentive plans
Shares authorized for issuance						7,200
Employee and Directors Stock Options
Stock-based compensation
Unrecognized compensation cost related to unvested stock options (in dollars) | $	$ 2,100		$ 2,100
Weighted-average periods over which unrecognized compensation cost recognized			1 year 6 months
Estimated forfeitures (as a percent)	1.50%		1.50%
Number of shares
Outstanding at the beginning of the period (in shares)			2,920
Outstanding at the end of the period (in shares)	2,843		2,843		2,920
Vested at the end of the period (in shares)	1,875		1,875
Exercisable at the end of the period (in shares)	1,875		1,875
Forfeited (in shares)			37
Expired (in shares)			(40)
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 11.64
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	11.67		11.67
Exercisable at the end of the period (in dollars per share) | $ / shares	$ 11.67		11.67
Forfeited (in dollars per share) | $ / shares			13.26
Expired (in dollars per share) | $ / shares			$ 11.51
Weighted-average remaining contractual term
Outstanding			6 years 8 months 12 days		7 years 7 months 6 days
Outstanding at the end of the period			6 years 8 months 12 days		7 years 7 months 6 days
Vested at the end of the period			6 years
Exercisable at the end of the period			6 years
Aggregate intrinsic value
Outstanding at the beginning of the period (in dollars) | $			$ 1,274
Outstanding at the end of the period (in dollars) | $	$ 2,311		2,311		$ 1,274
Vested at the end of the period (in dollars) | $	1,760		1,760
Exercisable at the end of the period (in dollars) | $	1,760		$ 1,760
Valuation assumptions
Dividend yield (as a percent)			0.00%
Restricted Stock
Stock-based compensation
Unrecognized compensation cost related to unvested restricted stock (in dollars) | $	$ 2,500		$ 2,500
Weighted-average periods over which unrecognized compensation cost recognized			1 year 3 months 18 days
Number of shares
Outstanding at the beginning of the period (in shares)			561
Granted (in shares)			99
Vested (in shares)			(204)
Forfeited (in shares)			(8)
Outstanding at the end of the period (in shares)	448		448		561
Weighted-average grant date fair value
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 10.58
Granted (in dollars per share) | $ / shares			11.10
Vested (in dollars per share) | $ / shares			11.80
Forfeited (in dollars per share) | $ / shares			13.38
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 10.09		$ 10.09		$ 10.58
Restricted Stock | Common Class A
Equity incentive plans
Shares available for issuance	2,900		2,900
Time Based Restricted Stock and Stock Option
Equity incentive plans
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)					1.60%
Risk-free interest rate, maximum (as a percent)					2.40%
Volatility, minimum (as a percent)					26.40%
Volatility, maximum (as a percent)					32.40%
Weighted-average expected term					6 years 2 months 12 days
Time Based Restricted Stock
Valuation assumptions
Vesting period			3 years
Event Based Restricted Stock and Stock Option | Common Class A
Equity incentive plans
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
Equity incentive plans
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		10
Event Based Stock Option
Aggregate intrinsic value
Unvested options	300		300
Event Based Restricted Stock
Number of shares
Outstanding at the end of the period (in shares)	200		200