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Income Taxes - Additional Information (Detail) (USD $)
12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2014
Sep. 30, 2014
Mar. 31, 2014
Dec. 31, 2011
Income Tax Contingency [Line Items]              
Unrecognized tax benefits, interest and penalties $ 6,200,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate     $ 6,200,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate      
Cash payments for income taxes 11,600,000us-gaap_IncomeTaxesPaid 21,100,000us-gaap_IncomeTaxesPaid 13,100,000us-gaap_IncomeTaxesPaid        
Operating loss carry forwards 600,000us-gaap_OperatingLossCarryforwards 300,000us-gaap_OperatingLossCarryforwards   600,000us-gaap_OperatingLossCarryforwards      
Operating loss carry forwards valuation allowance 0us-gaap_OperatingLossCarryforwardsValuationAllowance     0us-gaap_OperatingLossCarryforwardsValuationAllowance      
Foreign tax credit carry forwards 0us-gaap_DeferredTaxAssetsValuationAllowance 0us-gaap_DeferredTaxAssetsValuationAllowance 0us-gaap_DeferredTaxAssetsValuationAllowance 0us-gaap_DeferredTaxAssetsValuationAllowance     7,500,000us-gaap_DeferredTaxAssetsValuationAllowance
Unremitted earnings 743,000,000iosp_UnremittedEarningOfForeignSubsidiary 605,000,000iosp_UnremittedEarningOfForeignSubsidiary          
Foreign Income [Member]              
Income Tax Contingency [Line Items]              
Foreign tax credit carry forwards 800,000us-gaap_DeferredTaxAssetsValuationAllowance
/ us-gaap_TaxCreditCarryforwardAxis
= iosp_ForeignIncomeMember
4,500,000us-gaap_DeferredTaxAssetsValuationAllowance
/ us-gaap_TaxCreditCarryforwardAxis
= iosp_ForeignIncomeMember
  800,000us-gaap_DeferredTaxAssetsValuationAllowance
/ us-gaap_TaxCreditCarryforwardAxis
= iosp_ForeignIncomeMember
     
Unrecognized Tax Benefits [Member]              
Income Tax Contingency [Line Items]              
Reduction in unrecognized tax benefits         2,900,000us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis
= iosp_UnrecognizedTaxBenefitsMember
3,600,000us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis
= iosp_UnrecognizedTaxBenefitsMember
 
Unrecognized tax benefits       300,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromSettlementsWithTaxingAuthorities
/ us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis
= iosp_UnrecognizedTaxBenefitsMember
     
Interest and Penalties [Member]              
Income Tax Contingency [Line Items]              
Reduction in unrecognized tax benefits         $ 200,000us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis
= iosp_InterestAndPenaltiesMember
$ 400,000us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis
= iosp_InterestAndPenaltiesMember