Subsequent events - Narrative (Details) - Subsequent Event - USD ($) $ / shares in Units, $ in Millions		1 Months Ended
Jan. 31, 2019	Feb. 28, 2019
Subsequent Event [Line Items]
Common stock, issued and sold (in shares)	7,563,725
Sale price (in USD per share)	$ 30.20
Exercise of Underwriter (in shares)		843,725
Net proceeds from sale of stock	$ 224.1

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

No definition available.

Name:	us-gaap_SubsequentEventLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration