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Short-term Investments - Available for Sale Securities (Details) (USD $)
Share data in Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]      
Proceeds from sale of equity $ 0us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities $ 12,699,000us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities $ 31,506,000us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
Greenbrier [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Number of shares sold during period (in shares)   0.8arii_NumberOfSharesOfAvailableForSaleSecuritiesSold
/ dei_LegalEntityAxis
= arii_GreenbrierMember		  
Proceeds from sale of equity   12,700,000us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
/ dei_LegalEntityAxis
= arii_GreenbrierMember		  
Realized gains   $ 2,000,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ dei_LegalEntityAxis
= arii_GreenbrierMember