Annual and transition report of foreign private issuers pursuant to Section 13 or 15(d)

Operating Segment Information and Concentrations of Risk - Summary of Gross Sales by Major Customers (Detail)

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Operating Segment Information and Concentrations of Risk - Summary of Gross Sales by Major Customers (Detail) - USD ($)
$ in Millions
12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2017
Sales Revenue, Net | Geographic Concentration Risk      
Segment Reporting Information [Line Items]      
Gross sales percentage (as percent) 36.00% 22.00% 21.00%
Customer A | Accounts Receivable | Customer Concentration Risk      
Segment Reporting Information [Line Items]      
Gross sales percentage (as percent) 8.50% 9.00%  
Customer A | North America Fiber Cement | Sales Revenue, Net | Customer Concentration Risk      
Segment Reporting Information [Line Items]      
Net sales $ 260.5 $ 246.9 $ 226.0
Gross sales percentage (as percent) 10.40% 12.00% 10.30%