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Share-based Payments - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Sep. 01, 2020
Jan. 31, 2020
Nov. 30, 2020
Feb. 29, 2020
Nov. 30, 2019
Sep. 30, 2020
Aug. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Options outstanding (shares)     1,192,000        
Restricted Stock and Performance Share Awards              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Minimum age for eligibility under retirement vesting provision           60 years  
Minimum service period under retirement vesting provision           10 years  
Market Options              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Vesting period   4 years          
Stock options              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Award expiration period   10 years          
Duration of trailing average stock price       12 months      
Amended and Restated Acuity Brand, Inc. 2012 Omnibus Stock Compensation Incentive Plan | Stock options              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Options outstanding (shares)     1,200,000       900,000
Options granted (shares) 277,000   300,000        
Shares issued from options exercised in period (shares)     0   0    
Intrinsic value of options outstanding     $ 7.5        
Intrinsic value of options expected to vest     2.9        
Intrinsic value of options exercisable     4.5        
Unrecognized compensation cost     $ 23.6        
Recognition period for unrecognized compensation cost     2 years 6 months