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Asbestos - Asbestos Adjustments (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014 | Mar. 31, 2013
Change in estimates: |  |  |
Change in actuarial estimate - asbestos liability | $ (129.0)jhx_ChangeInActuarialEstimateAsbestosLiability | $ (340.3)jhx_ChangeInActuarialEstimateAsbestosLiability | $ (163.0)jhx_ChangeInActuarialEstimateAsbestosLiability
Change in actuarial estimate - insurance receivable | 16.6 jhx_ChangeInActuarialEstimateInsuranceReceivables | 31.2 jhx_ChangeInActuarialEstimateInsuranceReceivables | 27.9 jhx_ChangeInActuarialEstimateInsuranceReceivables
Change in estimate - AICF claims-handling costs | 1.1 jhx_ChangeInEstimateClaimsHandlingCosts | 0.9 jhx_ChangeInEstimateClaimsHandlingCosts | 5.9 jhx_ChangeInEstimateClaimsHandlingCosts
Subtotal - Change in estimates | (111.3)jhx_ChangesInEstimates | (308.2)jhx_ChangesInEstimates | (129.2)jhx_ChangesInEstimates
Recovery of Insurance Receivables |  | 15.2 us-gaap_FinancialGuaranteeInsuranceContractsPremiumsReceivableAdjustment | 11.9 us-gaap_FinancialGuaranteeInsuranceContractsPremiumsReceivableAdjustment
Gain on foreign currency exchange | 144.7 jhx_AsbestosAdjustmentGainLossOnForeignExchange | 97.2 jhx_AsbestosAdjustmentGainLossOnForeignExchange | 0.2 jhx_AsbestosAdjustmentGainLossOnForeignExchange
Total Asbestos Adjustments | $ 33.4 jhx_AsbestosAdjustments | $ (195.8)jhx_AsbestosAdjustments | $ (117.1)jhx_AsbestosAdjustments

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- Definition Gain (loss) on foreign exchange movements in the asbestos related assets and liabilities which are denominated in Australian dollars. + References No definition available. + Details
Name: | jhx_AsbestosAdjustmentGainLossOnForeignExchange
Namespace Prefix: | jhx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Comprised of changes in actuarial estimates received as of March 31 each year and the effect of foreign exchange movements in the asbestos related assets and liabilities which are denominated in Australian dollars. + References No definition available. + Details
Name: | jhx_AsbestosAdjustments
Namespace Prefix: | jhx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Change in actuarial estimate asbestos liability. + References No definition available. + Details
Name: | jhx_ChangeInActuarialEstimateAsbestosLiability
Namespace Prefix: | jhx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Change in actuarial estimate insurance receivables. + References No definition available. + Details
Name: | jhx_ChangeInActuarialEstimateInsuranceReceivables
Namespace Prefix: | jhx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Change in estimate claims handling costs. + References No definition available. + Details
Name: | jhx_ChangeInEstimateClaimsHandlingCosts
Namespace Prefix: | jhx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | jhx_ChangeInEstimatesAbstract
Namespace Prefix: | jhx_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Changes in estimates. + References No definition available. + Details
Name: | jhx_ChangesInEstimates
Namespace Prefix: | jhx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition This element represents adjustments to the premiums receivable on financial guarantee insurance contracts including, at a minimum, adjustments due to changes in the period of the contract, accretion of the discount on the premiums receivable, and other adjustments that impacted the reported premium receivable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 50 -Paragraph 3 -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290 + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsPremiumsReceivableAdjustment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration