ORGANIZATION AND BASIS OF PRESENTATION - Other Income And Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Entity Information [Line Items]
Non-service cost component of net benefit cost	$ (13.1)	$ (5.7)
Other	(6.3)	(4.9)
Other expense	(19.4)	(10.6)
Westar Energy
Entity Information [Line Items]
Non-service cost component of net benefit cost	(4.4)	(5.7)
Other	(6.2)	(4.9)
Other expense	(10.6)	(10.6)
KCP&L
Entity Information [Line Items]
Non-service cost component of net benefit cost	(5.1)	(6.7)
Other	0.1	(1.2)
Other expense	$ (5.0)	$ (7.9)