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Subsequent events (Details) - Subsequent Event
$ / shares in Units, $ in Millions
1 Months Ended
Apr. 30, 2018
USD ($)
$ / shares
shares
Subsequent Event [Line Items]  
Stock issued during period (in shares) 4,600,000
Shares issued (in dollars per share) | $ / shares $ 27.04
Proceeds from issuance of common stock | $ $ 117.9
Over-Allotment Option  
Subsequent Event [Line Items]  
Stock issued during period (in shares) 600,000