Stockholders' equity - Warrants (Details). $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares $ / item shares
Warrants
Issued (in shares)	12,100,000
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 exercised	3,408
Proceeds from exercise of warrant for Class A common stock | $	$ 20
Common Class A
Warrants
Warrants exercisable into number of shares	6,000,000
Number of shares entitled to warrant holders (as a percent)	0.5
Shares issued upon exercise of warrants	1,704
Proceeds from exercise of warrant for Class A common stock | $	$ 0