At-the-market offering (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions			12 Months Ended
	Aug. 08, 2019	Jun. 05, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 19, 2019
Common stock, aggregate offering price			$ 28	$ 22
Shares issued	2.7
stock issuance cost					$ 8,173
Net proceeds from issuance of common shares				$ (20)	$ 8,423
At-the-market offering
Shares issued		2.2
Shares issued price (in dollars per share)		$ 22.73
Gross proceeds		$ 49,700
stock issuance cost		1,700
Net proceeds from issuance of common shares		$ 48,000	$ 48,019
At-the-market offering | Maximum
Common stock, aggregate offering price						$ 50,000