At-the-market offering (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions			9 Months Ended
	Aug. 08, 2019	Jun. 05, 2019	Sep. 30, 2019	Sep. 30, 2018	Mar. 19, 2019	Dec. 31, 2018
Common Stock, Value, Issued			$ 28			$ 22
Shares issued	2.7
Gross Proceeds From Issuance Of Common Stock	$ 69,000
stock issuance cost	4,500
Net proceeds from issuance of common shares	$ 64,500			$ (53)
At-the-market offering
Common Stock, Value, Issued					$ 50,000
Shares issued		2.2
Shares issued (in dollars per share)		$ 22.73
Gross Proceeds From Issuance Of Common Stock		$ 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, Value, Issued					$ 50,000