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PENSION PLANS AND POST-RETIREMENT BENEFITS - Employee Savings Plans (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Defined contribution plan, cost $ 17.4 $ 17.6 $ 16.3
Evergy Kansas Central      
Defined Benefit Plan Disclosure [Line Items]      
Defined contribution plan, cost 9.6 9.6 9.9
Evergy Metro      
Defined Benefit Plan Disclosure [Line Items]      
Defined contribution plan, cost $ 7.8 $ 8.0 $ 8.3