Business Segment Information - Reconciliation of Gross Profit to Segment Adjusted Gross Profit (Detail) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and amortization	$ 216,261	$ 183,281	$ 154,903
Segment, Expenditure, Addition to Long-Lived Assets	249,766	212,944	183,812
Operating Segments \| Stormwater
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and amortization	126,387	102,574	81,602
Segment, Expenditure, Addition to Long-Lived Assets	157,381	175,918	128,313
Operating Segments \| Wastewater
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and amortization	80,423	74,068	69,121
Segment, Expenditure, Addition to Long-Lived Assets	30,509	13,733	17,882
Operating Segments \| Other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and amortization	9,451	6,639	4,180
Segment, Expenditure, Addition to Long-Lived Assets	$ 61,876	$ 23,293	$ 37,617