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Stock-Based Compensation (Employee Stock Purchase Plan) (Details) (Employee Stock Purchase Plan 1987 [Member])
In Millions, unless otherwise specified
Mar. 27, 2015
Employee Stock Purchase Plan 1987 [Member]
  
Employee Stock Purchase Plan [Line Items]  
Part of fair market value of common stock used to determine purchase price for share under ESPP 85.00%altr_ShareBasedCompensationArrangementPercentageOfPurchasePriceLowerThanFairMarketValueOfCommonStock
/ altr_EmployeeStockPurchasePlanEsppDisclosuresByPlanAxis
= altr_EmployeeStockPurchasePlan1987Member
Shares available for future issuance under Employee Stock Purchase Plan 3.4altr_SharesAvailableForFutureIssuanceUnderEmployeeStockPurchasePlan
/ altr_EmployeeStockPurchasePlanEsppDisclosuresByPlanAxis
= altr_EmployeeStockPurchasePlan1987Member