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Capital structure - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended				3 Months Ended	12 Months Ended
	Sep. 30, 2019	Aug. 31, 2019	Jan. 31, 2019	Apr. 30, 2018	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Capital structure
Common stock, issued and sold (in shares)	2,475,248		7,563,725		63,926
Net proceeds from sale of stock	$ 97,000		$ 224,200		$ 2,600
Sale price (in USD per share)	$ 46.60		$ 30.20		$ 46.60
Issuance of common stock related to equity offering (in shares)				4,600,000
Share price (in USD per share)				$ 27.04
Net proceeds from public offering				$ 117,900		$ 323,756	$ 117,916	$ 0
Common stock, authorized shares (in shares)						125,000,000	125,000,000
Common stock
Capital structure
Sale price (in USD per share)	$ 40.40				$ 40.40
Common stock, authorized shares (in shares)						125,000,000
Over-Allotment Option
Capital structure
Common stock, issued and sold (in shares)	371,287		843,725
Issuance of common stock related to equity offering (in shares)				600,000
Maximum
Capital structure
Net proceeds from sale of stock		$ 125,000

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- Definition

The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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