Significant accounting policies Accounts receivable and concentration of credit risk (Details) - USD ($) $ in Millions	12 Months Ended
	Jan. 31, 2018	Jan. 31, 2017
Concentration Risk [Line Items]
Retention receivable	$ 3.2
Accounts Receivable, Additional Narrative Disclosure	outstanding for several years as of January 31, 2018. The Company completed all of its deliverables in 2015, and has been engaged in ongoing active efforts to collect this amount, and has recently received an updated acknowledgment of the outstanding balances and assurances of payment from the customer. As a result, the Company did not reserve any allowance against this amount as of January 31, 2018. However, if the Company’s efforts to collect on this account are not successful in fiscal 2018, then the Company may be required to recognize an allowance for all, or substantially all, of any such then uncollected amounts in the future.
Accounts Receivable [Member]
Concentration Risk [Line Items]
One customer's account receivable	$ 5.4
Retention receivable	$ 3.7
Customer concentration risk, percentage	34.90%	33.20%