Retirement Benefits - Summary of Change in Projected Benefit Obligations and Change in Fair Value of Plan Assets (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Compensation And Retirement Disclosure [Abstract]
Accumulated Benefit Obligation, April 30	$ 20,683	$ 19,061
Change in Projected Benefit Obligations
Projected benefit obligations, beginning of year	19,061	17,328
Interest cost	906	942	959
Actuarial loss	1,610	1,611
Actual benefits paid	(894)	(820)
Projected benefit obligations, end of year	20,683	19,061	17,328
Change in Plan Assets
Fair value of plan assets, beginning of year	14,007	14,979
Actual return (loss) on plan assets	1,302	(554)
Employer contributions	1,000	402	847
Actual benefits paid	(894)	(820)
Fair value of plan assets, end of year	15,415	14,007	14,979
Funded status - under	(5,268)	(5,054)
Amounts Recognized in the Consolidated Balance Sheets consist of:
Noncurrent assets	0
Noncurrent liabilities	(5,268)	(5,054)
Net amount recognized	(5,268)	(5,054)
Amounts recognized in accumulated other comprehensive income (loss) consist of:
Net actual loss	11,078	10,658
Deferred tax benefit	(4,310)	(4,146)
After-tax actuarial loss	$ 6,768	$ 6,512
Weighted-Average Assumptions Used to Determine Benefit Obligations at April 30
Discount rate	4.25%	4.75%
Rate of compensation increase
Weighted-Average Assumptions Used to Determine Net Periodic Benefit Cost for Years Ended April 30
Discount rate	4.75%	5.60%
Expected long-term return on plan assets	8.50%	8.75%
Rate of compensation increase