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Acquired In-Place Leases and Above/Below Market Leases (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Acquired Finite-Lived Intangible Assets [Line Items]      
Amortization of acquired above and below market leases $ 2.8us-gaap_AmortizationOfAboveAndBelowMarketLeases $ 2.4us-gaap_AmortizationOfAboveAndBelowMarketLeases $ (0.2)us-gaap_AmortizationOfAboveAndBelowMarketLeases
Weighted-average amortization period of below market leases 8 years 6 months    
Acquired lease, below market renewal options $ 17.5aat_FiniteLivedIntangibleLiabilitiesBelowMarketLeasesRenewalOptions    
Weighted-average amortization period of acquired leases prior to the commencement of renewal option 10 years 9 months    
In-place leases      
Acquired Finite-Lived Intangible Assets [Line Items]      
Weighted-average amortization period of acquired leases 2 years 5 months    
Above Market Leases      
Acquired Finite-Lived Intangible Assets [Line Items]      
Weighted-average amortization period of acquired leases 1 year 5 months