Discontinued Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Statement of operations
Net loss from discontinued operations	$ (2,508)	$ (4,154)	$ (6,427)
EES Business | Discontinued Operations
Statement of operations
Net Sales	37,899	35,933	30,360
Cost of sales	30,890	29,008	23,831
Gross margin	7,009	6,925	6,529
Selling, general and administrative	7,825	8,895	9,614
Research and development	3,526	4,577	7,251
Other (expense), net	(27)
Other income, net		7	28
Income from discontinued operations before income taxes	(4,369)	(6,540)	(10,308)
Benefit for income taxes	(1,861)	(2,386)	(3,881)
Net loss from discontinued operations	$ (2,508)	$ (4,154)	$ (6,427)