Stockholders' equity - (Details) $ / shares in Units, shares in Thousands, $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	May 16, 2016 shares	Sep. 30, 2016 USD ($) item $ / shares shares	Sep. 30, 2016 USD ($) item $ / shares shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) item $ / shares shares	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) $ / shares shares
Stock-based compensation
Stock-based compensation expense (in dollars) | $			$ 500	$ 1,400	$ 2,500	$ 4,100
Common Class A
Equity incentive plans
Shares authorized for issuance		7,200	7,200		7,200
Employee and Directors Stock Options
Stock-based compensation
Unrecognized compensation cost related to unvested stock options (in dollars) | $		$ 1,300	$ 1,300		$ 1,300
Weighted-average periods over which unrecognized compensation cost recognized					1 year 10 months 24 days
Estimated forfeitures (as a percent)		1.50%	1.50%		1.50%
Number of shares
Outstanding at the beginning of the period (in shares)					2,043
Granted (in shares)					30
Outstanding at the end of the period (in shares)		2,060	2,060		2,060		2,043
Vested at the end of the period (in shares)		1,487	1,487		1,487
Exercisable at the end of the period (in shares)		1,487	1,487		1,487
Exercised (in shares)					(13)
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share) | $ / shares					$ 11.49
Granted (in dollars per share) | $ / shares					13.64
Outstanding at the end of the period (in dollars per share) | $ / shares		$ 11.53	$ 11.53		11.53		$ 11.49
Vested at the end of the period (in dollars per share) | $ / shares		11.52	11.52		11.52
Exercisable at the end of the period (in dollars per share) | $ / shares		$ 11.52	$ 11.52		11.52
Exercised (in dollars per share) | $ / shares					$ 10.60
Weighted-average remaining contractual term
Outstanding					6 years 10 months 24 days		7 years 7 months 6 days
Outstanding at the end of the period					6 years 10 months 24 days		7 years 7 months 6 days
Vested at the end of the period					6 years 8 months 12 days
Exercisable at the end of the period					6 years 8 months 12 days
Aggregate intrinsic value
Outstanding at the beginning of the period (in dollars) | $					$ 6,740
Outstanding at the end of the period (in dollars) | $		$ 3,466	$ 3,466		3,466		$ 6,740
Vested at the end of the period (in dollars) | $		2,597	2,597		2,597
Exercisable at the end of the period (in dollars) | $		2,597	2,597		$ 2,597
Weighted-average grant date fair value of options granted (in dollars per share) | $ / shares					$ 3.96
Valuation assumptions
Dividend yield (as a percent)					0.00%
Restricted Stock
Stock-based compensation
Unrecognized compensation cost related to unvested restricted stock (in dollars) | $		$ 800	$ 800		$ 800
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)					494
Granted (in shares)					79
Vested (in shares)					(308)
Outstanding at the end of the period (in shares)		265	265		265		494
Weighted-average grant date fair value
Outstanding at the beginning of the period (in dollars per share) | $ / shares					$ 9.79
Granted (in dollars per share) | $ / shares					11.41
Vested (in dollars per share) | $ / shares					10.78
Outstanding at the end of the period (in dollars per share) | $ / shares		$ 9.13	$ 9.13		$ 9.13		$ 9.79
Restricted Stock | Common Class A
Equity incentive plans
Additional of number of shares authorized to issue	3,200
Shares available for issuance		4,000	4,000		4,000
Time Based Restricted Stock and Stock Option
Equity incentive plans
Number of equal annual installments for vesting of awards | item		3	3		3
Time Based Stock Option | Black Scholes Pricing Model
Valuation assumptions
Risk-free interest rate, minimum (as a percent)						1.76%
Risk-free interest rate, maximum (as a percent)						2.12%
Risk-free interest rates (as a percent)					1.60%
Volatility, minimum (as a percent)						25.80%
Volatility, maximum (as a percent)						29.50%
Volatility (as a percent)					26.37%
Weighted-average expected term					6 years	6 years 3 months 18 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		10
Event Based Stock Option
Aggregate intrinsic value
Unvested options		300	300		300
Event Based Restricted Stock
Number of shares
Outstanding at the end of the period (in shares)		200	200		200