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RATE MATTERS AND REGULATION - Narrative (Details)
$ in Millions
1 Months Ended 12 Months Ended
Sep. 30, 2019
USD ($)
Rate
May 31, 2019
USD ($)
Apr. 30, 2019
USD ($)
Dec. 31, 2019
USD ($)
Rate
Aug. 31, 2019
Rate
Mar. 31, 2019
Rate
Dec. 31, 2018
USD ($)
Sep. 30, 2018
Rate
Regulatory Proceedings [Line Items]                
Regulatory liabilities       $ 2,311.6     $ 2,329.0  
Pension and post-retirement costs                
Regulatory Proceedings [Line Items]                
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided       735.4        
Regulatory asset representing differences between pension and post retirement costs for GAAP and for ratemaking       288.4        
Analog meter unrecovered investment                
Regulatory Proceedings [Line Items]                
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided       $ 21.6        
Amortization period       5 years        
Kansas property tax surcharge                
Regulatory Proceedings [Line Items]                
Amortization period       1 year        
Deferred customer programs                
Regulatory Proceedings [Line Items]                
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided       $ 10.2        
Transmission delivery charge                
Regulatory Proceedings [Line Items]                
Amortization period       1 year        
Fuel recovery mechanisms                
Regulatory Proceedings [Line Items]                
Amortization period       1 year        
Evergy Kansas Central                
Regulatory Proceedings [Line Items]                
Regulatory liabilities       $ 1,174.8     1,121.3  
Evergy Kansas Central | Jeffrey Energy Center                
Regulatory Proceedings [Line Items]                
Jointly owned utility plant, proportionate ownership share | Rate 8.00%     8.00% 8.00% 8.00%   8.00%
Evergy Kansas Central | Pension and post-retirement costs                
Regulatory Proceedings [Line Items]                
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided       $ 359.9        
Regulatory asset representing differences between pension and post retirement costs for GAAP and for ratemaking       (23.6)        
Evergy Kansas Central | Analog meter unrecovered investment                
Regulatory Proceedings [Line Items]                
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided       $ 21.6        
Amortization period       5 years        
Evergy Metro                
Regulatory Proceedings [Line Items]                
Regulatory liabilities       $ 803.6     847.1  
Evergy Metro | Pension and post-retirement costs                
Regulatory Proceedings [Line Items]                
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided       312.9        
Regulatory asset representing differences between pension and post retirement costs for GAAP and for ratemaking       131.1        
Evergy Metro | Deferred customer programs                
Regulatory Proceedings [Line Items]                
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided       8.3        
Purchase power agreement                
Regulatory Proceedings [Line Items]                
Regulatory liabilities       $ 7.4     8.8  
Regulatory liability amortization period       5 years        
Purchase power agreement | Evergy Kansas Central                
Regulatory Proceedings [Line Items]                
Regulatory liabilities       $ 7.4     8.8  
Purchase power agreement | Evergy Metro                
Regulatory Proceedings [Line Items]                
Regulatory liabilities       0.0     0.0  
Fuel recovery mechanisms                
Regulatory Proceedings [Line Items]                
Regulatory liabilities       $ 34.1     0.0  
Regulatory liability amortization period       1 year        
Fuel recovery mechanisms | Evergy Kansas Central                
Regulatory Proceedings [Line Items]                
Regulatory liabilities       $ 30.2     0.0  
Fuel recovery mechanisms | Evergy Metro                
Regulatory Proceedings [Line Items]                
Regulatory liabilities       $ 0.0     $ 0.0  
La Cygne Station | Evergy Kansas South Inc                
Regulatory Proceedings [Line Items]                
VIE ownership percentage       50.00%        
Transmission delivery charge | Evergy Kansas Central | Kansas Corporation Commission                
Regulatory Proceedings [Line Items]                
Annual revenue decrease approved     $ 7.7          
Transmission delivery charge | Evergy Metro | Kansas Corporation Commission                
Regulatory Proceedings [Line Items]                
Annual revenue decrease approved   $ 8.3            
2019 RECA Recovery Request | Kansas Corporation Commission                
Regulatory Proceedings [Line Items]                
Pre tax loss to operating and maintenance expense $ 8.4              
2019 RECA Recovery Request | Evergy Kansas Central | Kansas Corporation Commission                
Regulatory Proceedings [Line Items]                
Pre tax loss to operating and maintenance expense       $ 8.4        
2018 Rate Case Proceedings | Evergy Kansas Central | Kansas Corporation Commission                
Regulatory Proceedings [Line Items]                
Refund percent on earnings over approved return on equity       0.50        
Approved return on equity, percentage       9.30%        
2018 Rate Case Proceedings | Evergy Metro | Kansas Corporation Commission                
Regulatory Proceedings [Line Items]                
Refund percent on earnings over approved return on equity       0.5        
Approved return on equity, percentage       9.30%        
FERC 2020 TFR Proceeding | Evergy Kansas Central | Federal Energy Regulatory Commission                
Regulatory Proceedings [Line Items]                
Annual transmissions revenue estimated amount of decrease over prior year       $ 6.8        
FERC 2020 TFR Proceeding | Evergy Metro | Federal Energy Regulatory Commission                
Regulatory Proceedings [Line Items]                
Annual transmissions revenue estimated amount of decrease over prior year       (1.7)        
FERC 2019 TFR Proceeding | Evergy Kansas Central | Federal Energy Regulatory Commission                
Regulatory Proceedings [Line Items]                
Annual transmissions revenue estimated amount of decrease over prior year       (11.2)        
FERC 2019 TFR Proceeding | Evergy Metro | Federal Energy Regulatory Commission                
Regulatory Proceedings [Line Items]                
Annual transmissions revenue estimated amount of decrease over prior year       (2.8)        
FERC 2018 TFR Proceeding | Evergy Kansas Central | Federal Energy Regulatory Commission                
Regulatory Proceedings [Line Items]                
Annual transmissions revenue estimated amount of decrease over prior year       2.3        
FERC 2018 TFR Proceeding | Evergy Metro | Federal Energy Regulatory Commission                
Regulatory Proceedings [Line Items]                
Annual transmissions revenue estimated amount of decrease over prior year       3.7        
2018 MPSC OPC MECG Petition                
Regulatory Proceedings [Line Items]                
Annual estimated loss contingency portion not accrued (up to)       12.0        
2018 MPSC OPC MECG Petition | Revenues Probable Of Refund                
Regulatory Proceedings [Line Items]                
Regulatory liabilities       $ 10.2