Revenue recognition - Contract Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Jul. 31, 2018	Apr. 30, 2018
Change in Contract with Customer, Liability [Roll Forward]
Beginning balance	$ 570	$ 1,967
Revenue recognized during the period for uncompleted contracts from the prior period	(422)	(1,810)
New contracts entered into that are uncompleted at the end of the current period	398	413
Ending balance	$ 546	$ 570