SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Income Expense Net [Line Items]
Non-service cost component of net benefit cost	$ (55.6)	$ (47.8)	$ (20.0)
Other	(21.3)	(30.9)	(19.1)
Other expense	(76.9)	(78.7)	(39.1)
Evergy Kansas Central
Other Income Expense Net [Line Items]
Non-service cost component of net benefit cost	(20.1)	(23.5)	(20.0)
Other	(20.0)	(23.3)	(19.1)
Other expense	(40.1)	(46.8)	(39.1)
Evergy Metro
Other Income Expense Net [Line Items]
Non-service cost component of net benefit cost	(20.9)	(25.9)	(42.7)
Other	(0.5)	(5.0)	(8.1)
Other expense	$ (21.4)	$ (30.9)	$ (50.8)