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Credit Facility and Long-Term Debt (Credit Facility) (Details) (Line of Credit [Member], USD $)
0 Months Ended
Jun. 29, 2012
Mar. 27, 2015
Line of Credit [Member]
   
Debt Instrument [Line Items]    
Debt instrument, Term 5 years 0 months 0 days  
Line of credit facility, maximum borrowing capacity $ 250,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
 
Increase in borrowing capacity of line of credit 250,000,000altr_IncreaseInBorrowingCapacityOfLineOfCredit
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
 
Revolving Credit Facility   $ 0us-gaap_LinesOfCreditCurrent
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember