Employee benefit plans - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Shares (in thousands)
Outstanding at beginning of period (shares)	3,963
Granted (shares)	631
Exercised (shares)	(7)
Forfeited/Cancelled (shares)	(320)
Outstanding at end of period (shares)	4,267	3,963
Weighted-average exercise price
Outstanding at beginning of period (in dollars per share)	$ 10.16
Granted (usd per share)	4.08
Exercised (usd per share)	0.94
Outstanding at end of period (in dollars per share)	$ 9.25	$ 10.16
Weighted Average Remaining Contractual Term (in years)	6 years 5 months 23 days	6 years 4 months 6 days
Aggregate intrinsic value (in thousands)	$ 157	$ 374
Vested and Expected to Vest (shares)	4,267
Vested and Expected to Vest - Weighted Average Exercise Price (in dollars per share)	$ 9.25
Vested and Expected to Vest- Weighted Average Remaining Contractual Term	6 years 5 months 23 days
Vested and Expected to Vest - Aggregate Intrinsic Value	$ 157
Exercisable (shares)	2,934
Exercisable - Weighted average exercise price (in dollars per share)	$ 11.01
Exercisable - Weighted Average Remaining Contractual Term	5 years 3 months
Exercisable - Aggregate intrinsic value	$ 157
Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price	$ 10.46

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.