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Fair Value - Narrative (Details) (USD $)
Dec. 31, 2014
Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis    
Long-term debt, carrying value $ 1,741,834,000us-gaap_LongTermDebt $ 1,742,122,000us-gaap_LongTermDebt
Credit valuation adjustment to total fair value percentage 1.00%atw_FairValueInputCreditValuationAdjustmentToTotalFairValuePercentage  
Senior Notes | 6.5% Senior Notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis    
Long-term debt, carrying value 656,834,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= atw_SixPointFivePercentSeniorNotesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 657,122,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= atw_SixPointFivePercentSeniorNotesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Principal amount 650,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= atw_SixPointFivePercentSeniorNotesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		  
Long-term debt, fair value $ 658,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_DebtInstrumentAxis
= atw_SixPointFivePercentSeniorNotesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember