Retirement Benefits - Summary of Fair Value Assets in Defined Benefit Pension Plans (Detail) - USD ($) $ in Thousands	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 18,540	$ 17,198	$ 15,817
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,599	6,466
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,298	121
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18,540	17,198
Level 1 [Member] | Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,929	8,060
Level 1 [Member] | Small/Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,405	1,728
Level 1 [Member] | International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,889	823
Level 1 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,599	6,466
Level 1 [Member] | Liquid Alternatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	420
Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 4,298	$ 121