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Share-based Payments - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 8 Months Ended 9 Months Ended 12 Months Ended
Jan. 31, 2020
May 31, 2020
Feb. 29, 2020
May 31, 2020
May 31, 2020
Aug. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Options outstanding (shares)   917,000   917,000 917,000  
Options granted (shares) 500,000          
Performance stock awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Performance share units outstanding (shares)   67,000   67,000 67,000  
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    
Service Options            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period 3 years          
Options granted (shares) 400,000          
Market Options            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period 4 years          
Options granted (shares) 100,000          
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)   900,000   900,000 900,000 400,000
Options granted (shares)   0     500,000  
Shares issued from options exercised in period (shares)         522 0
Intrinsic value of options outstanding   $ 1.5   $ 1.5 $ 1.5  
Intrinsic value of options expected to vest   0.0   0.0 0.0  
Intrinsic value of options exercisable   1.5   1.5 1.5  
Unrecognized compensation cost   $ 16.8   $ 16.8 $ 16.8  
Recognition period for unrecognized compensation cost         2 years  
Three-year vesting performance shares approved in beginning of fiscal 2020 | Performance stock awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period         3 years  
Two-year vesting performance shares granted in second quarter of 2020 | Performance stock awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     2 years