v3.10.0.1
Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2018
Feb. 15, 2019
Jun. 30, 2018
Document Information [Line Items]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2018    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Trading Symbol AAT    
Entity Registrant Name AMERICAN ASSETS TRUST, INC.    
Entity Central Index Key 0001500217    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer Yes    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Large Accelerated Filer    
Entity Emerging Growth Company false    
Entity Small Business false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   47,325,507  
Entity Public Float     $ 1,559,900,000
American Assets Trust, L.P.      
Document Information [Line Items]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2018    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Entity Registrant Name AMERICAN ASSETS TRUST, L.P.    
Entity Central Index Key 0001509570    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Non-accelerated Filer    
Entity Emerging Growth Company false    
Entity Small Business false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   0  
Entity Public Float     $ 0