Changes in Accounting Estimates (Details) (USD $)	3 Months Ended								12 Months Ended
	Apr. 30, 2014	Jan. 25, 2014	Oct. 26, 2013	Jul. 27, 2013	Apr. 30, 2013	Jan. 26, 2013	Oct. 27, 2012	Jul. 28, 2012	Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2012
Changes in accounting estimates
Income from continuing operations									$ 14,896,000	$ 10,773,000	$ 43,538,000
Net income	8,057,000	11,216,000	1,655,000	(7,210,000)	(795,000)	3,869,000	8,738,000	(1,386,000)	13,718,000	10,426,000	30,451,000
Basic earnings per share (in dollars per share)	$ 0.36	$ 0.50	$ 0.07	$ (0.32)	$ (0.04)	$ 0.17	$ 0.40	$ (0.06)	$ 0.61	$ 0.47	$ 1.40
Diluted earnings per share (in dollars per share)	$ 0.35	$ 0.49	$ 0.07	$ (0.32)	$ (0.04)	$ 0.17	$ 0.39	$ (0.06)	$ 0.60	$ 0.47	$ 1.36
Revision of estimate of total costs required to complete the contracts | Cumulative catch-up adjustment
Changes in accounting estimates
Income from continuing operations									362,000	1,768,000	3,065,000
Net income									$ 223,000	$ 1,081,000	$ 1,870,000
Basic earnings per share (in dollars per share)									$ 0.01	$ 0.05	$ 0.09
Diluted earnings per share (in dollars per share)									$ 0.01	$ 0.05	$ 0.08