FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		10 Months Ended
	Sep. 30, 2019	Mar. 31, 2020	Mar. 31, 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	$ 69,800,000
Notional amount	500,000,000.0			$ 500,000,000.0
Other comprehensive loss	69,800,000	$ 0	$ 13,800,000
Interest expense		(96,200,000)	(91,100,000)
Income tax expense (benefit)		10,100,000	$ 9,300,000
Accumulated other comprehensive loss over the next twelve months		5,200,000
Senior Notes Due 2029 Evergy | Senior Notes
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt issued	$ 800,000,000.0			$ 800,000,000.0
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		1,700,000
Income tax expense (benefit)		$ 400,000