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

Asbestos - Asbestos Adjustments (Detail)

v2.4.1.9
Asbestos - Asbestos Adjustments (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
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 16.6jhx_ChangeInActuarialEstimateInsuranceReceivables 31.2jhx_ChangeInActuarialEstimateInsuranceReceivables 27.9jhx_ChangeInActuarialEstimateInsuranceReceivables
Change in estimate - AICF claims-handling costs 1.1jhx_ChangeInEstimateClaimsHandlingCosts 0.9jhx_ChangeInEstimateClaimsHandlingCosts 5.9jhx_ChangeInEstimateClaimsHandlingCosts
Subtotal - Change in estimates (111.3)jhx_ChangesInEstimates (308.2)jhx_ChangesInEstimates (129.2)jhx_ChangesInEstimates
Recovery of Insurance Receivables   15.2us-gaap_FinancialGuaranteeInsuranceContractsPremiumsReceivableAdjustment 11.9us-gaap_FinancialGuaranteeInsuranceContractsPremiumsReceivableAdjustment
Gain on foreign currency exchange 144.7jhx_AsbestosAdjustmentGainLossOnForeignExchange 97.2jhx_AsbestosAdjustmentGainLossOnForeignExchange 0.2jhx_AsbestosAdjustmentGainLossOnForeignExchange
Total Asbestos Adjustments $ 33.4jhx_AsbestosAdjustments $ (195.8)jhx_AsbestosAdjustments $ (117.1)jhx_AsbestosAdjustments