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Changes in accumulated other comprehensive income (loss) - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]    
Amounts reclassified from accumulated other comprehensive income, pension and other postretirement plans, net of tax $ (0.1)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax $ 0.1us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax
Amounts reclassified from accumulated other comprehensive income, pension and other postretirement plans, tax $ (0.1)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax $ 0.1us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax