Revenue recognition - Contract Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Jul. 31, 2018	Apr. 30, 2018
Change in Contract with Customer, Asset [Roll Forward]
Beginning balance	$ 1,834	$ 1,502
Costs and gross profit recognized during the period for uncompleted contracts from the prior period	(1,395)	(1,085)
Costs and deferred gross profit incurred on uncompleted contracts not billed at the end of the current period	1,575	1,417
Ending balance	$ 2,014	$ 1,834