Retirement Benefits - Summary of Fair Value Assets in Defined Benefit Pension Plans (Detail) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 15,415	$ 14,007	$ 14,979
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,475	4,547
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,186	43
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,415	14,007
Level 1 [Member] | Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,875	5,225
Level 1 [Member] | Small/Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,535	1,329
Level 1 [Member] | Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	216	1,134
Level 1 [Member] | Real Estate/Commodities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	128	527
Level 1 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,475	4,547
Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,186	43
Level 1 [Member] | International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,202
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 2 [Member] | Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 2 [Member] | Small/Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 2 [Member] | Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 2 [Member] | Real Estate/Commodities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 2 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 2 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 3 [Member] | Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 3 [Member] | Small/Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 3 [Member] | Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 3 [Member] | Real Estate/Commodities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 3 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Level 3 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets