FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended		10 Months Ended
	Sep. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Payment for settlement of interest rate swap accounted for as a cash flow hedge	$ 69.8
Notional amount	500.0					$ 500.0
Other comprehensive loss	69.8	$ 0.0	$ 21.2	$ 0.0	$ 35.0
Interest expense		99.5	95.4	195.7	186.5
Income tax expense (benefit)		33.7	$ 24.4	43.8	$ 33.7
Accumulated other comprehensive loss over the next twelve months		5.2		5.2
Senior Notes Due 2029 Evergy | Senior Notes
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt issued	$ 800.0					$ 800.0
Interest rate	2.90%					2.90%
Debt instrument, term						10 years
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Interest expense		1.8		3.5
Income tax expense (benefit)		$ (2.8)		$ (3.2)