Summary of business and significant accounting policies (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Jul. 31, 2020	Oct. 31, 2020	Oct. 31, 2020	Oct. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Principal payments on long-term debt			$ 223,438	$ 0
Term Loan Facility | Line of Credit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Principal payments on long-term debt		$ 200,000	$ 200,000
Follow on equity offering
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock, shares (in shares)	5,290,000
Offering price per share (in usd per share)	$ 56.00
Net proceeds from follow on offering	$ 286,800
Underwriting discounts and commissions	8,900
Other offering expenses payable	600
Principal payments on long-term debt	200,000
Follow on equity offering | Term Loan Facility | Line of Credit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Principal payments on long-term debt	$ 200,000