Stockholders' equity - Warrants (Details) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) item $ / shares $ / item shares	Sep. 30, 2016 USD ($)
Warrants
Exercise price of warrants per half share | $ / item	6.00
Minimum number of warrants exercisable by holder (in shares)	2
Exercise price of warrants (in dollars per share) | $ / shares	$ 12.00
Number of warrants that may be called for redemption (in shares)	7,600,000
Number of warrants repurchased	0
Number of warrants exercised	190,749
Proceeds from exercise of warrant for Class A common stock | $	$ 211	$ 420
Common Class A
Warrants
Issued (in shares)	12,100,000
Outstanding (in shares)	6,100,000
Number of shares entitled to warrant holders (as a percent)	0.5
Warrants exercisable into number of shares	6,000,000
Trigger price of stock in order to provide for redemption of warrants at option of the Company (in dollars per share) | $ / shares	$ 18.00
Number of trading days through which last sales price of common stock is reported for warrant redemption | item	20
Period of aggregate number of trading days through which last sales price of common stock is reported for warrant redemption	30 days
Shares issued upon exercise of warrants	23,911