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Operating Segment Information and Concentrations of Risk - Summary of Gross Sales by Major Customers (Detail) $ in Millions | 12 Months Ended
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Mar. 31, 2020 USD ($) Customer | Mar. 31, 2019 USD ($) | Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items] |  |  |
JHX_Number of Customer | Customer | 1 |  |
jhx_Percentage Of Gross Sales Per Customer Maximum | 10.00% |  |
Sales Revenue, Net | Geographic Concentration Risk |  |  |
Segment Reporting Information [Line Items] |  |  |
Gross sales percentage (as percent) | 34.00% | 36.00% | 22.00%
Customer A | Accounts Receivable | Customer Concentration Risk |  |  |
Segment Reporting Information [Line Items] |  |  |
Gross sales percentage (as percent) | 6.00% | 8.50% |
Customer A | North America Fiber Cement | Sales Revenue, Net | Customer Concentration Risk |  |  |
Segment Reporting Information [Line Items] |  |  |
Net sales | $ | $ 306.0 | $ 260.5 | $ 246.9
Gross sales percentage (as percent) | 12.00% | 10.40% | 12.00%

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