Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Oct. 31, 2017		Oct. 31, 2016		Oct. 31, 2017		Oct. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance						$ 1,083,439
Ending balance		$ 1,132,014				1,132,014
Accumulated Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(39,275)		$ (49,467)		(55,359)		$ (36,339)
Unrealized (losses) gains arising during the period		(4,019)		(5,131)		12,065		(18,259)
Ending balance		(43,294)		(54,598)		(43,294)		(54,598)
Accumulated Defined Benefit Plan Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(14,775)	[1]	(21,110)	[2]	(15,127)	[1]	(21,572)	[2]
Reclassification of realized net losses to net income		352	[1]	465	[2]	704	[1]	927	[2]
Ending balance		(14,423)	[1]	(20,645)	[2]	(14,423)	[1]	(20,645)	[2]
Unrealized (Losses) Gains on Interest Rate Swap
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[3]	(641)				(578)
Unrealized (losses) gains arising during the period	[3]	322				88
Reclassification of realized net losses to net income	[3]	134				305
Ending balance	[3]	(185)				(185)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(54,691)		(70,577)		(71,064)		(57,911)
Unrealized (losses) gains arising during the period		(3,697)		(5,131)		12,153		(18,259)
Reclassification of realized net losses to net income		486		465		1,009		927
Ending balance		$ (57,902)		$ (75,243)		$ (57,902)		$ (75,243)

[1] The tax effect on the reclassifications of realized net losses was $0.2 million and $0.5 million for the three and six months ended October 31, 2017, respectively.

[2] The tax effect on the reclassifications of realized net losses was $0.3 million and $0.6 million for the three and six months ended October 31, 2016, respectively.

[3] The tax effect on unrealized gains was $0.2 million and $0.1 million for the three and six months ended October 31, 2017, respectively. The tax effect on the reclassification of realized net losses to net income was $0.1 million and $0.2 million for the three and six months ended October 31, 2017, respectively.