Comprehensive Income (Loss) - Changes in Each Component of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Oct. 31, 2018		Oct. 31, 2017		Oct. 31, 2018		Oct. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance						$ 1,216,607
Ending balance		$ 1,173,238				1,173,238
Accumulated Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(46,961)		$ (39,275)		(32,399)		$ (55,359)
Unrealized (losses) gains arising during the period		(12,511)		(4,019)		(27,073)		12,065
Ending balance		(59,472)		(43,294)		(59,472)		(43,294)
Accumulated Defined Benefit Plan Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(11,196)	[1]	(14,775)	[2]	(9,073)	[1]	(15,127)	[2]
Reclassification of realized net losses (gains) to net income		273	[1]	352	[2]	546	[1]	704	[2]
Effect of adoption of accounting standard	[1]					(2,396)
Ending balance		(10,923)	[1]	(14,423)	[2]	(10,923)	[1]	(14,423)	[2]
Unrealized Gains (Losses) on Interest Rate Swap
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		1,669	[3]	(641)	[4]	1,337	[3]	(578)	[4]
Unrealized (losses) gains arising during the period		193	[3]	322	[4]	342	[3]	88	[4]
Reclassification of realized net losses (gains) to net income		(48)	[3]	134	[4]	(64)	[3]	305	[4]
Effect of adoption of accounting standard	[3]					199
Ending balance		1,814	[3]	(185)	[4]	1,814	[3]	(185)	[4]
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(56,488)		(54,691)		(40,135)		(71,064)
Unrealized (losses) gains arising during the period		(12,318)		(3,697)		(26,731)		12,153
Reclassification of realized net losses (gains) to net income		225		486		482		1,009
Effect of adoption of accounting standard						(2,197)
Ending balance		$ (68,581)		$ (57,902)		$ (68,581)		$ (57,902)

[1] The tax effect on the reclassifications of realized net losses was $0.1 million and $0.2 million for the three and six months ended October 31, 2018, respectively.

[2] 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.

[3] The tax effect on unrealized gains was $0.1 million for both the three and six months ended October 31, 2018, respectively.

[4] 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.