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Stock-Based Compensation (Details) (USD $)
 0 Months Ended 3 Months Ended 12 Months Ended
May 13, 2014
May 05, 2014
May 06, 2013
Jul. 30, 2012
Mar. 27, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Shares reserved for future issuance under the Equity Plan         25,700,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant         17,800,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant      
Restricted Stock Units And Performance Based Restricted Stock Units [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Vesting period for stock-based awards granted under the Equity Plan (in years)         4 years 0 months 0 days      
RSUs and PRSUs, Number of Shares                
Outstanding, December 31, 2014         5,506,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Grants         92,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Vested         (222,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Forfeited/Cancelled         (79,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Outstanding, March 27, 2015         5,297,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Vested and expected to vest, March 27, 2015         4,792,000altr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOtherThanOptionsVestedAndExpectedToVestOutstandingNumber
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
RSUs and PRSUs, Weighted-Average Grant-Date Fair Market Value Per Share (in dollars per share):                
Outstanding, December 31, 2014         34.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Grants         34.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Vested         38.94us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Forfeited/Cancelled         33.88us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Outstanding, March 27, 2015         33.80us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Vested and expected to vest, March 27, 2015         33.80altr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		      
Outstanding, Weighted-Average Remaining Contractual Term (in Years)         1 year 2 months 0 days      
Outstanding, Aggregate Intrinsic Value         235,136,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		 [1]      
Vested and expected to vest, Weighted-Average Remaining Contractual Term (in Years)         1 year 2 months 0 days      
Vested and expected to vest, Aggregate Intrinsic Value         212,699,000altr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestInPeriodAggregateIntrinsicValue
/ us-gaap_AwardTypeAxis
= altr_RestrictedStockUnitsAndPerformanceBasedRestrictedStockUnitsMember		 [1]      
Employee Stock Option [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Contractual term for stock-based awards granted under the Equity plan (in years)         10 years 0 months 0 days      
Stock Options, Number of Shares                
Outstanding, December 31, 2014         2,074,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Grants         0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Exercises         (104,000)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Forfeited/Cancelled/Expired         (17,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Outstanding, March 27, 2015         1,953,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Exercisable, March 27, 2015         1,524,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Vested and expected to vest, March 27, 2015         1,931,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Stock Options, Weighted-Average Exercise Price Per Share (in dollars per share):                
Outstanding, December 31, 2014         32.60us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Exercises         20.40us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Forfeited/Cancelled/Expired         32.37us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Outstanding, March 27, 2015         33.26us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Exercisable, March 27, 2015         32.75us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Vested and expected to vest, March 27, 2015         33.25us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Outstanding, Weighted-Average Remaining Contractual Term (in Years)         5 years 1 month 0 days      
Exercisable, Weighted-Average Contractual Term (in Years)         4 years 7 months 0 days      
Vested and expected to vest, Weighted-Average Remaining Contractual Term (in Years)         5 years 1 month 0 days      
Outstanding, Aggregate Intrinsic Value         21,741,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		 [2]      
Exercisable, Aggregate Intrinsic Value         17,732,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		 [2]      
Vested and expected to vest, Aggregate Intrinsic Value         21,518,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		 [2]      
Intrinsic value of stock options exercised         1,700,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Consideration recorded from exercise of stock options         2,100,000us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Performance-based Restricted Stock Units (Market-Based PRSUs) [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Vesting period for stock-based awards granted under the Equity Plan (in years)         3 years 0 months 0 days      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage, Maximum         200.00%altr_SharebasedCompensationArrangementbySharebasedPaymentAwardAwardVestingRightsPercentageMaximum
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage, Minimum         0.00%altr_SharebasedCompensationArrangementbySharebasedPaymentAwardAwardVestingRightsPercentageMinimum
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		      
RSUs and PRSUs, Number of Shares                
Grants           303,260us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 262,647us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 66,489us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
RSUs and PRSUs, Weighted-Average Grant-Date Fair Market Value Per Share (in dollars per share):                
Grants $ 31.13us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 $ 31.23us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 $ 33.03us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 $ 41.18us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		        
[1] Aggregate intrinsic value represents the closing price per share of our stock on March 27, 2015, multiplied by the number of RSUs and market-based PRSUs outstanding or vested and expected to vest as of March 27, 2015.
[2] For those stock options with an exercise price below the closing price per share on March 27, 2015, aggregate intrinsic value represents the difference between the exercise price and the closing price per share of our common stock on March 27, 2015, multiplied by the number of stock options outstanding, exercisable, or vested and expected to vest as of March 27, 2015.