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Pension Plans - Defined Benefit Pension Plan (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Equity Securities [Member] |  |  |
Plan asset allocation by category (%): |  |  |
Total plan asset allocation by category | 35.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember |  |
Debt Securities [Member] |  |  |
Plan asset allocation by category (%): |  |  |
Total plan asset allocation by category | 65.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember |  |
United Kingdom Plan [Member] |  |  |
Plan net pension charge/(credit): |  |  |
Service cost | $ 1.7 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_PlanNameAxis = iosp_UkPlanMember | $ 1.6 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_PlanNameAxis = iosp_UkPlanMember | $ 1.6 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_PlanNameAxis = iosp_UkPlanMember
Interest cost on PBO | 34.7 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_PlanNameAxis = iosp_UkPlanMember | 31.3 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_PlanNameAxis = iosp_UkPlanMember | 32.2 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_PlanNameAxis = iosp_UkPlanMember
Expected return on plan assets | (37.2)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_PlanNameAxis = iosp_UkPlanMember | (35.5)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_PlanNameAxis = iosp_UkPlanMember | (34.1)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_PlanNameAxis = iosp_UkPlanMember
Amortization of prior service credit | (1.3)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_PlanNameAxis = iosp_UkPlanMember | (1.3)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_PlanNameAxis = iosp_UkPlanMember | (1.3)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_PlanNameAxis = iosp_UkPlanMember
Amortization of actuarial net losses | 5.4 us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss / us-gaap_PlanNameAxis = iosp_UkPlanMember | 6.2 us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss / us-gaap_PlanNameAxis = iosp_UkPlanMember | 3.2 us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss / us-gaap_PlanNameAxis = iosp_UkPlanMember
Net pension charge | $ 3.3 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_PlanNameAxis = iosp_UkPlanMember | $ 2.3 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_PlanNameAxis = iosp_UkPlanMember | $ 1.6 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_PlanNameAxis = iosp_UkPlanMember
Plan assumptions at December 31, (%): |  |  |
Discount rate | 3.55%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate / us-gaap_PlanNameAxis = iosp_UkPlanMember | 4.40%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate / us-gaap_PlanNameAxis = iosp_UkPlanMember | 4.15%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate / us-gaap_PlanNameAxis = iosp_UkPlanMember
Inflation rate | 2.15%iosp_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostInflationRate / us-gaap_PlanNameAxis = iosp_UkPlanMember | 2.55%iosp_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostInflationRate / us-gaap_PlanNameAxis = iosp_UkPlanMember | 2.20%iosp_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostInflationRate / us-gaap_PlanNameAxis = iosp_UkPlanMember
Rate of return on plan assets | 4.05%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / us-gaap_PlanNameAxis = iosp_UkPlanMember | 4.85%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / us-gaap_PlanNameAxis = iosp_UkPlanMember | 4.90%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / us-gaap_PlanNameAxis = iosp_UkPlanMember
Plan asset allocation by category (%): |  |  |
Total plan asset allocation by category | 100.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_PlanNameAxis = iosp_UkPlanMember | 100.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_PlanNameAxis = iosp_UkPlanMember | 100.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_PlanNameAxis = iosp_UkPlanMember
United Kingdom Plan [Member] | Equity Securities [Member] |  |  |
Plan assumptions at December 31, (%): |  |  |
Rate of return on plan assets | 6.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember | 7.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember | 7.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember
Plan asset allocation by category (%): |  |  |
Total plan asset allocation by category | 32.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember | 35.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember | 33.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember
United Kingdom Plan [Member] | Debt Securities [Member] |  |  |
Plan assumptions at December 31, (%): |  |  |
Rate of return on plan assets | 2.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember | 3.40%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember | 3.65%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember
Plan asset allocation by category (%): |  |  |
Total plan asset allocation by category | 63.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember | 61.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember | 61.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember / us-gaap_PlanNameAxis = iosp_UkPlanMember
United Kingdom Plan [Member] | Cash [Member] |  |  |
Plan asset allocation by category (%): |  |  |
Total plan asset allocation by category | 5.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_PlanNameAxis = iosp_UkPlanMember | 4.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_PlanNameAxis = iosp_UkPlanMember | 6.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_PlanNameAxis = iosp_UkPlanMember

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- Definition Defined benefit plan assumptions used calculating net periodic benefit cost inflation rate. + References No definition available. + Details
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- Definition The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The interest rate used to adjust for the time value of money. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512013 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512136 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512171 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Amount included in accumulated other comprehensive income (AOCI) related to gain (loss) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position. + References No definition available. + Details
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- Definition The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Service Cost (Component of Net Periodic Pension Cost) -URI http://asc.fasb.org/extlink&oid=6525008 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Target allocation of investments of this type to total plan assets presented on a weighted average basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 + Details
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