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Pension and Postretirement Expense Level 4 (Details) - Amounts Recognized in Other Comprehensive Income (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax $ (77)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax $ 68us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax $ (108)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax
United States Pension Plans of US Entity, Defined Benefit [Member]      
Actuarial losses 17us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
   
Prior Service Cost from Plan Amendments 0msc_PriorServiceCostFromPlanAmendments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
   
Amortization of prior service cost 0us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
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= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
0us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
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= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
0us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
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= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Actuarial Gain (Loss), before Tax 8us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeLossNetGainLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
   
Loss (Gain) recognized in other comprehensive income 9msc_LossGainRecognizedInOtherComprehensiveIncome
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
   
Deferred income taxes 0msc_DeferredIncomeTaxes
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
   
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax 9us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
   
Foreign Pension Plans, Defined Benefit [Member]      
Actuarial losses 77us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
   
Prior Service Cost from Plan Amendments (2)msc_PriorServiceCostFromPlanAmendments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
   
Amortization of prior service cost 0us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
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= us-gaap_ForeignPensionPlansDefinedBenefitMember
(1)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
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= us-gaap_ForeignPensionPlansDefinedBenefitMember
(1)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Actuarial Gain (Loss), before Tax 3us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeLossNetGainLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
   
Loss (Gain) recognized in other comprehensive income 72msc_LossGainRecognizedInOtherComprehensiveIncome
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
   
Deferred income taxes (2)msc_DeferredIncomeTaxes
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
   
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax 70us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
   
United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]      
Actuarial losses (3)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
   
Prior Service Cost from Plan Amendments 0msc_PriorServiceCostFromPlanAmendments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
   
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Actuarial Gain (Loss), before Tax (2)us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeLossNetGainLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
   
Loss (Gain) recognized in other comprehensive income (1)msc_LossGainRecognizedInOtherComprehensiveIncome
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
   
Deferred income taxes 0msc_DeferredIncomeTaxes
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
   
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax (1)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
   
Foreign Postretirement Benefit Plans, Defined Benefit [Member]      
Actuarial losses 1us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
   
Prior Service Cost from Plan Amendments (2)msc_PriorServiceCostFromPlanAmendments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
   
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Actuarial Gain (Loss), before Tax 0us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeLossNetGainLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
   
Loss (Gain) recognized in other comprehensive income (1)msc_LossGainRecognizedInOtherComprehensiveIncome
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
   
Deferred income taxes 0msc_DeferredIncomeTaxes
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
   
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax $ (1)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember