FAIR VALUE MEASUREMENTS - Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	$ 58.4	$ 16.6	$ (29.9)	$ 57.1
Nuclear decommissioning trust - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	53.3	13.3	(36.0)	51.2
Nuclear decommissioning trust - debt securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	2.3	2.3	5.3	4.4
Rabbi trusts - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	2.8	1.0	0.8	1.5
Evergy Kansas Central
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	24.7	9.7	(15.5)	28.2
Evergy Kansas Central | Nuclear decommissioning trust - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	22.1	8.8	(16.0)	26.0
Evergy Kansas Central | Rabbi trusts - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	2.6	0.9	0.5	2.2
Evergy Metro
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	33.5	6.8	(14.7)	29.6
Evergy Metro | Nuclear decommissioning trust - equity securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	31.2	4.5	(20.0)	25.2
Evergy Metro | Nuclear decommissioning trust - debt securities
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gain (losses)	$ 2.3	$ 2.3	$ 5.3	$ 4.4