FAIR VALUE MEASUREMENTS - Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	$ (88.3)	$ 40.5
Nuclear decommissioning trust - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	(89.3)	37.9
Nuclear decommissioning trust - debt securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	3.0	2.1
Rabbi trusts - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	(2.0)	0.5
Evergy Kansas Central
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	(40.2)	18.5
Evergy Kansas Central | Nuclear decommissioning trust - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	(38.1)	17.2
Evergy Kansas Central | Rabbi trusts - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	(2.1)	1.3
Evergy Metro
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	(48.2)	22.8
Evergy Metro | Nuclear decommissioning trust - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	(51.2)	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)	$ 3.0	$ 2.1