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Stock-Based Compensation - Summary of Stock Options Available for Grant (Detail) - shares | 12 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant [Roll Forward] |  |
Shares Available for Grant, Beginning balance | 25,458,910 | 27,238,814
Granted | 1,714,094 | 1,779,904
Shares Available for Grant, Ending balance | 23,744,816 | 25,458,910

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- Definition Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Available For Grant Granted In Period + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant [Roll Forward] + References No definition available. + Details
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Namespace Prefix: | jhx_
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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