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Income Taxes (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2008
Dec. 08, 2011
Mar. 29, 2013
Jan. 23, 2012
Income taxes Textual [Line Items]              
Valuation allowance $ 32,805,000us-gaap_DeferredTaxAssetsValuationAllowance $ 24,498,000us-gaap_DeferredTaxAssetsValuationAllowance          
Deferred Charge, Current 2,200,000altr_DeferredChargeCurrent 2,200,000altr_DeferredChargeCurrent          
Deferred Charge, Non-current 16,800,000altr_DeferredChargeNoncurrent 18,900,000altr_DeferredChargeNoncurrent          
Income taxes paid       18,000,000us-gaap_IncomeTaxesPaid      
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions 6,900,000us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions 30,300,000us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions          
Other Tax Expense (Benefit) 900,000us-gaap_OtherTaxExpenseBenefit 27,700,000us-gaap_OtherTaxExpenseBenefit          
Research and development tax credit carryforwards   10,600,000us-gaap_TaxCreditCarryforwardAmount          
Unrecognized tax benefit that would impact effective tax rate 341,100,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 301,300,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate          
Unrecognized Tax Benefits, Penalties And Interest Expense, Net 4,200,000altr_UnrecognizedTaxBenefitsPenaltiesAndInterestExpenseBenefitNet 100,000altr_UnrecognizedTaxBenefitsPenaltiesAndInterestExpenseBenefitNet (6,000,000)altr_UnrecognizedTaxBenefitsPenaltiesAndInterestExpenseBenefitNet        
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense 5,800,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense            
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense, Reduction Resulting from Lapse of Applicable Statute of Limitations 1,600,000us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations            
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued 45,800,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 48,800,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued          
Business Combination, Indemnification Assets, Amount as of Acquisition Date 6,500,000us-gaap_BusinessCombinationIndemnificationAssetsAmountAsOfAcquisitionDate 6,500,000us-gaap_BusinessCombinationIndemnificationAssetsAmountAsOfAcquisitionDate          
Undistributed Earnings of Foreign Subsidiaries 3,400,000,000us-gaap_UndistributedEarningsOfForeignSubsidiaries            
Domestic Tax Authority [Member]              
Income taxes Textual [Line Items]              
Operating Loss Carryforwards 8,000,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		            
Annual limitation on use of tax credit 900,000altr_AnnualLimitationOnTaxCreditFromAcquiredCompany
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		            
California Research and Development Tax Credits [Member]              
Income taxes Textual [Line Items]              
Research and development tax credit carryforwards 32,900,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_TaxCreditCarryforwardAxis
= altr_CaliforniaResearchAndDevelopmentTaxCreditsMember		            
Federal Research Tax Credit Carryforward [Member]              
Income taxes Textual [Line Items]              
Research and development tax credit carryforwards 1,300,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_TaxCreditCarryforwardAxis
= us-gaap_ResearchMember		            
Tax Return Examination Years 2002 through 2004 [Member]              
Income taxes Textual [Line Items]              
Liability (refund) adjustment from settlement with taxing authority         19,800,000us-gaap_IncomeTaxExaminationLiabilityRefundAdjustmentFromSettlementWithTaxingAuthority
/ us-gaap_TaxPeriodAxis
= altr_TaxReturnExaminationYears2002Through2004Member		    
Tax Return Examination Years 2004 through 2007 [Member]              
Income taxes Textual [Line Items]              
Liability (refund) adjustment from settlement with taxing authority           (6,800,000)us-gaap_IncomeTaxExaminationLiabilityRefundAdjustmentFromSettlementWithTaxingAuthority
/ us-gaap_TaxPeriodAxis
= altr_TaxReturnExaminationYears2004through2007Member		  
Tax Return Examination Years 2005 through 2007 [Member]              
Income taxes Textual [Line Items]              
Liability (refund) adjustment from settlement with taxing authority             $ 21,400,000us-gaap_IncomeTaxExaminationLiabilityRefundAdjustmentFromSettlementWithTaxingAuthority
/ us-gaap_TaxPeriodAxis
= altr_TaxReturnExaminationYears2005Through2007Member