Stockholders' equity - Narrative (Details) $ / shares in Units, $ in Thousands			12 Months Ended
	Nov. 25, 2019 USD ($) $ / shares shares	Jun. 11, 2018 USD ($) $ / shares shares	Dec. 31, 2019 vote $ / shares	Sep. 12, 2019 $ / shares shares
Class of Stock [Line Items]
Vote per share | vote			1
Dividends declared (usd per share) | $ / shares			$ 0
Note Purchase Agreement | Senior Notes
Class of Stock [Line Items]
Number of shares called by warrants (in shares) | shares				810,357
Common stock warrant exercise price (in dollars per share) | $ / shares				$ 6.72
Public Offering
Class of Stock [Line Items]
Number of shares issued (in shares) | shares	5,500,000	5,750,000
Price per share (usd per share) | $ / shares	$ 9.75	$ 22.25
Consideration received on transaction | $	$ 38,747	$ 59,917
Sale of stock discounts | $		3,678
Deferred offering costs | $	308	$ 373
Fees and commissions | $	$ 2,383
The Company | Public Offering
Class of Stock [Line Items]
Number of shares issued (in shares) | shares	4,250,000	2,875,000
Certain Stockholders | Public Offering
Class of Stock [Line Items]
Number of shares issued (in shares) | shares	1,250,000	2,875,000