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

Income Taxes - Reconciliation of Unrecognized Tax Benefits and Interest and Penalties (Detail)

v2.4.1.9
Income Taxes - Reconciliation of Unrecognized Tax Benefits and Interest and Penalties (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2013
Income Tax Disclosure [Abstract]      
Unrecognized tax benefits, Beginning balance $ 0.5us-gaap_UnrecognizedTaxBenefits $ 1.5us-gaap_UnrecognizedTaxBenefits $ 2.6us-gaap_UnrecognizedTaxBenefits
Unrecognized tax benefits, Additions for tax positions of the current year 4.2us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions 0.1us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions 0.1us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
Unrecognized tax benefits, Additions for tax positions of prior year 0.2us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 0.1us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 2.6us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
Unrecognized tax benefits, Expiration of statute of limitations     (2.8)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations
Unrecognized tax benefits, Settlements paid during the current period   (1.2)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities  
Unrecognized tax benefits, Other reductions for the tax positions of prior periods     (1.0)jhx_UnrecognizedTaxBenefitsOtherDecreasesResultingFromPriorPeriodTaxPositions
Unrecognized tax benefits, Ending Balance 4.9us-gaap_UnrecognizedTaxBenefits 0.5us-gaap_UnrecognizedTaxBenefits 1.5us-gaap_UnrecognizedTaxBenefits
Interest and Penalties, Beginning balance   0.1us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 0.9us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued
Interest and Penalties, Additions for tax positions of the current year 0.1jhx_InterestAndPenaltiesAdditionsForTaxPositionsOfCurrentYear    
Interest and Penalties, Additions for tax positions of prior year 0.2jhx_InterestAndPenaltiesAdditionsDeletionsForTaxPositionsOfPriorYear   (0.1)jhx_InterestAndPenaltiesAdditionsDeletionsForTaxPositionsOfPriorYear
Interest and Penalties, Expiration of statute of limitations     (0.7)jhx_TaxExpensesOnInterestAndPenaltiesRelatedToLapsesInStatutesOfLimitationsAndAuditSettlements
Interest and Penalties, Settlements paid during the current period   0jhx_InterestAndPenaltiesSettlementsPaidDuringCurrentPeriod  
Interest and Penalties, Other reductions for the tax positions of prior periods   (0.1)jhx_InterestAndPenaltiesOtherDecreasesResultingFromPriorPeriodTaxPositions  
Interest and Penalties, Ending Balance $ 0.3us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued   $ 0.1us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued