Goodwill and Intangible Assets - Carrying Amount of Goodwill by Reportable Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Goodwill [Roll Forward]
Beginning balance	$ 720,223	$ 617,183
Acquisitions	322,167	103,676
Reallocation due to change in segments	0
Currency translation	326	636
Ending balance	1,042,716	720,223
Pipe
Goodwill [Roll Forward]
Beginning balance	65,766	65,766
Acquisitions	0	0
Reallocation due to change in segments	(65,766)
Currency translation	0	0
Ending balance	0	65,766
Infiltrator
Goodwill [Roll Forward]
Beginning balance	599,517	495,841
Acquisitions	0	103,676
Reallocation due to change in segments	(599,517)
Currency translation	0	0
Ending balance	0	599,517
International
Goodwill [Roll Forward]
Beginning balance	9,805	10,441
Acquisitions	0	0
Reallocation due to change in segments	(9,805)
Currency translation	0	636
Ending balance	0	9,805
Allied Products & Other
Goodwill [Roll Forward]
Beginning balance	45,135	45,135
Acquisitions	0	0
Reallocation due to change in segments	(45,135)
Currency translation	0	0
Ending balance	0	45,135
Stormwater
Goodwill [Roll Forward]
Beginning balance	0	0
Acquisitions	(321,612)	0
Reallocation due to change in segments	120,706
Currency translation	(326)	0
Ending balance	442,644	0
Wastewater
Goodwill [Roll Forward]
Beginning balance	0	0
Acquisitions	(555)	0
Reallocation due to change in segments	599,517
Currency translation	0	0
Ending balance	$ 600,072	$ 0

Amount, after accumulated impairment loss, of asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

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