FAIR VALUE MEASUREMENTS - Long-Term Debt (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Book Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	$ 9,768.7	$ 9,627.3
Book Value | Fair Value, Measurements, Recurring | Variable Interest Entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	0.0	18.8
Fair Value | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	10,967.0	11,274.2
Fair Value | Fair Value, Measurements, Recurring | Level 2 | Variable Interest Entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	$ 0.0	19.1
Evergy Kansas Central | Fair Value, Measurements, Recurring | NAV | Alternative investments fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Length of Settlement	65 days
Evergy Kansas Central | Fair Value, Measurements, Recurring | NAV | Real estate securities fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Length of Settlement	65 days
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	$ 3,933.5	3,931.5
Evergy Kansas Central | Book Value | Fair Value, Measurements, Recurring | Variable Interest Entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	0.0	18.8
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	4,552.9	4,801.7
Evergy Kansas Central | Fair Value | Fair Value, Measurements, Recurring | Level 2 | Variable Interest Entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	0.0	19.1
Evergy Metro | Book Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	2,924.5	2,923.0
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	3,440.2	3,591.2
Long-Term debt fair value adjustment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt gross	$ 100.3	$ 110.4