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Share-Based Compensation - Summary of Company's Employee Share Option Activity (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Number of options, Outstanding, Beginning balance 7,665,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber    
Number of options, Granted 177,600us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross    
Number of options, Exercised (2,939,863)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised    
Number of options, Forfeited (35,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod    
Number of options, Outstanding, Ending balance 4,868,237us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber 7,665,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber  
Number of options, Vested and expected to vest, Ending balance 4,818,877us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber    
Number of options, Exercisable, Ending balance 2,149,037us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber    
Weighted average exercise price, Outstanding, Beginning balance $ 2.20us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice    
Weighted average exercise price, Granted $ 2.20us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice    
Weighted average exercise price, Exercised $ 2.20us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice    
Weighted average exercise price, Forfeited $ 2.20us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice    
Weighted average exercise price, Outstanding, Ending balance $ 2.20us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice $ 2.20us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice  
Weighted average exercise price, Vested or expected to vest $ 2.20us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice    
Weighted average exercise price, Exercisable $ 2.20us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice    
Weighted average grant date fair value, Outstanding, Beginning balance $ 2.65us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue    
Weighted average grant date fair value, Granted $ 34.57us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue   $ 2.36us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Weighted average grant date fair value, Outstanding, Ending balance $ 3.77us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue $ 2.65us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue  
Weighted average grant date fair value, Vested or expected to vest $ 3.73athm_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestWeightedAverageGrantDateFairValue    
Weighted average grant date fair value, Exercisable $ 2.52athm_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageGrantDateFairValue    
Weighted average remaining contractual term 6 years 10 months 21 days 7 years 8 months 27 days  
Weighted average remaining contractual term, Vested or expected to vest 6 years 10 months 13 days    
Weighted average remaining contractual term, Exercisable 6 years 6 months 11 days    
Aggregate intrinsic value of share options, Outstanding, Beginning balance $ 263,617us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue    
Aggregate intrinsic value of share options, Outstanding, Ending balance 166,299us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue 263,617us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue  
Aggregate intrinsic value of share options, Vested or expected to vest 164,613us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue    
Aggregate intrinsic value of share options, Exercisable $ 73,411us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1