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Debt - Additional Information (Detail) (USD $)
0 Months Ended 3 Months Ended 9 Months Ended
Sep. 05, 2014
Nov. 28, 2014
Nov. 28, 2014
Feb. 28, 2014
Nov. 29, 2013
Debt Disclosure [Line Items]          
Letters of credit outstanding   $ 27,700,000us-gaap_LettersOfCreditOutstandingAmount 27,700,000us-gaap_LettersOfCreditOutstandingAmount    
Carrying value of Corporation's non-publicly traded debt   325,000,000us-gaap_DebtInstrumentCarryingAmount 325,000,000us-gaap_DebtInstrumentCarryingAmount 340,000,000us-gaap_DebtInstrumentCarryingAmount 350,000,000us-gaap_DebtInstrumentCarryingAmount
Amendment [Member]          
Debt Disclosure [Line Items]          
Credit facility amendment description (i) reduce the interest rates applicable to the term loan and revolving loans, (ii) eliminate the LIBOR floor interest rate used in the determination of interest charged on Eurodollar revolving loans, (iii) reduce the commitment fee applicable to unused revolving commitments and (iv) reset the usage term of the general restricted payment basket with effect from September 5, 2014.        
Unamortized financing fees written off   1,900,000us-gaap_WriteOffOfDeferredDebtIssuanceCost
/ us-gaap_StatementScenarioAxis
= am_AmendmentMember
     
Capitalized new fees   1,100,000us-gaap_DebtIssuanceCosts
/ us-gaap_StatementScenarioAxis
= am_AmendmentMember
     
Current Portion of Term Loan [Member]          
Debt Disclosure [Line Items]          
Interest on credit facility borrowings     2.70%am_CreditFacilityCurrentInterestRate
/ us-gaap_CreditFacilityAxis
= am_TermLoanFacilityMember
   
Revolving Credit Facility [Member]          
Debt Disclosure [Line Items]          
Interest on credit facility borrowings     2.70%am_CreditFacilityCurrentInterestRate
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
   
Current borrowing capacity   250,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
250,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
   
Accounts Receivable Facility [Member]          
Debt Disclosure [Line Items]          
Interest on credit facility borrowings     0.60%am_CreditFacilityCurrentInterestRate
/ us-gaap_CreditFacilityAxis
= am_AccountsReceivableFacilityMember
   
Available financing of receivables   50,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= am_AccountsReceivableFacilityMember
50,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= am_AccountsReceivableFacilityMember
   
Publicly Traded [Member]          
Debt Disclosure [Line Items]          
Carrying value of Corporation's publicly traded debt   225,200,000us-gaap_UnsecuredDebt
/ us-gaap_LongtermDebtTypeAxis
= am_PubliclyTradedMember
225,200,000us-gaap_UnsecuredDebt
/ us-gaap_LongtermDebtTypeAxis
= am_PubliclyTradedMember
225,200,000us-gaap_UnsecuredDebt
/ us-gaap_LongtermDebtTypeAxis
= am_PubliclyTradedMember
225,200,000us-gaap_UnsecuredDebt
/ us-gaap_LongtermDebtTypeAxis
= am_PubliclyTradedMember
Publicly Traded [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]          
Debt Disclosure [Line Items]          
Fair value of Corporation's publicly and non-publicly traded debt   238,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_LongtermDebtTypeAxis
= am_PubliclyTradedMember
238,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_LongtermDebtTypeAxis
= am_PubliclyTradedMember
234,700,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_LongtermDebtTypeAxis
= am_PubliclyTradedMember
228,800,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_LongtermDebtTypeAxis
= am_PubliclyTradedMember
Non Publicly Traded [Member]          
Debt Disclosure [Line Items]          
Carrying value of Corporation's non-publicly traded debt   373,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= am_NonPubliclyTradedMember
373,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= am_NonPubliclyTradedMember
344,500,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= am_NonPubliclyTradedMember
427,300,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= am_NonPubliclyTradedMember
Non Publicly Traded [Member] | Level 2 [Member]          
Debt Disclosure [Line Items]          
Fair value of Corporation's publicly and non-publicly traded debt   $ 369,600,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_LongtermDebtTypeAxis
= am_NonPubliclyTradedMember
369,600,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_LongtermDebtTypeAxis
= am_NonPubliclyTradedMember
$ 344,500,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_LongtermDebtTypeAxis
= am_NonPubliclyTradedMember
$ 427,300,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_LongtermDebtTypeAxis
= am_NonPubliclyTradedMember