FAIR VALUE MEASUREMENTS - Gains (Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	$ 45.2	$ 82.2	$ (53.6)
Nuclear decommissioning trust - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	45.5	74.0	(54.1)
Nuclear decommissioning trust - debt securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	5.3	5.1	(0.5)
Rabbi trusts - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	(5.6)	3.1	1.0
Evergy Kansas Central
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	15.8	36.5	(30.8)
Evergy Kansas Central | Nuclear decommissioning trust - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	21.9	33.3	(31.8)
Evergy Kansas Central | Rabbi trusts - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	(6.1)	3.2	1.0
Evergy Metro
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	28.9	45.8	(23.2)
Evergy Metro | Nuclear decommissioning trust - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	23.6	40.7	(20.7)
Evergy Metro | Nuclear decommissioning trust - debt securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	$ 5.3	$ 5.1	$ (2.5)