FAIR VALUE MEASUREMENTS - Long-Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Book Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	$ 9,629.9	$ 8,997.8
Fair Value | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	11,126.4	9,750.2
Evergy Kansas Central | Book Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	3,931.1	3,686.1
Evergy Kansas Central | Fair Value | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	4,711.5	4,078.8
Evergy Metro | Book Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	2,922.7	2,525.0
Evergy Metro | Fair Value | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	3,555.6	2,932.2
Long-Term debt fair value adjustment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt gross	114.1	125.5
Variable Interest Entities | Book Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	18.8	51.1
Variable Interest Entities | Fair Value | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	19.3	51.5
Variable Interest Entities | Evergy Kansas Central | Book Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	18.8	51.1
Variable Interest Entities | Evergy Kansas Central | Fair Value | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	$ 19.3	$ 51.5