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Debt Obligations Level 4 (Details) - Scheduled Maturities (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Debt and Commitments Maturities [Line Items]      
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months $ 98us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths    
Operating Leases, Future Minimum Payments Due, Next Twelve Months 35us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent    
Capital Leases, Future Minimum Payments Due, Next Twelve Months 2us-gaap_CapitalLeasesFutureMinimumPaymentsDueCurrent    
Long-term Debt, Maturities, Repayments of Principal in Year Two 34us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo    
Capital Leases, Future Minimum Payments Due in Two Years 2us-gaap_CapitalLeasesFutureMinimumPaymentsDueInTwoYears    
Operating Leases, Future Minimum Payments, Due in Two Years 30us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears    
Long-term Debt, Maturities, Repayments of Principal in Year Three 42us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree    
Capital Leases, Future Minimum Payments Due in Three Years 2us-gaap_CapitalLeasesFutureMinimumPaymentsDueInThreeYears    
Operating Leases, Future Minimum Payments, Due in Three Years 22us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears    
Long-term Debt, Maturities, Repayments of Principal in Year Four 1,261us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour    
Capital Leases, Future Minimum Payments Due in Four Years 2us-gaap_CapitalLeasesFutureMinimumPaymentsDueInFourYears    
Operating Leases, Future Minimum Payments, Due in Four Years 16us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears    
Long-term Debt, Maturities, Repayments of Principal in Year Five 0us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive    
Capital Leases, Future Minimum Payments Due in Five Years 2us-gaap_CapitalLeasesFutureMinimumPaymentsDueInFiveYears    
Operating Leases, Future Minimum Payments, Due in Five Years 8us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears    
Long-term Debt, Maturities, Repayments of Principal after Year Five 2,387us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive    
Capital Leases, Future Minimum Payments Due Thereafter 5us-gaap_CapitalLeasesFutureMinimumPaymentsDueThereafter    
Operating Leases, Future Minimum Payments, Due Thereafter 17us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter    
Capital Leases, Future Minimum Payments Due 15us-gaap_CapitalLeasesFutureMinimumPaymentsDue    
Operating Leases, Future Minimum Payments Due 128us-gaap_OperatingLeasesFutureMinimumPaymentsDue    
Amount representing interest (6)us-gaap_CapitalLeasesFutureMinimumPaymentsInterestIncludedInPayments    
Present value of minimum payments 9us-gaap_CapitalLeasesFutureMinimumPaymentsPresentValueOfNetMinimumPayments    
Rental Expense under Operating Leases 36us-gaap_LeaseAndRentalExpense 36us-gaap_LeaseAndRentalExpense 36us-gaap_LeaseAndRentalExpense
Non-affiliated Debt [Member]      
Debt and Commitments Maturities [Line Items]      
Total minimum payments $ 3,822msc_TotalMinimumPayment
/ msc_TypeOfCommitmentAxis
= msc_NonAffiliatedDebtMember