ORGANIZATION AND BASIS OF PRESENTATION - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash paid for (received from):
Interest, net of amounts capitalized	$ 59.3	$ 33.0
Income taxes, net of refunds	(0.1)	(0.2)
Non-cash investing transactions:
Property, plant and equipment additions	47.1	29.8
Non-cash financing transactions:
Issuance of stock for compensation and reinvested dividends	0.5	0.1
Variable Interest Entities
Cash paid for (received from):
Interest, net of amounts capitalized	1.0	1.3
Westar Energy
Cash paid for (received from):
Interest, net of amounts capitalized	32.6	33.0
Income taxes, net of refunds	0.0	(0.2)
Non-cash investing transactions:
Property, plant and equipment additions	27.1	29.8
Non-cash financing transactions:
Issuance of stock for compensation and reinvested dividends	0.0	0.1
Westar Energy | Variable Interest Entities
Cash paid for (received from):
Interest, net of amounts capitalized	1.0	1.3
KCP&L
Cash paid for (received from):
Interest, net of amounts capitalized	19.6	22.7
Non-cash investing transactions:
Property, plant and equipment additions	$ 15.9	$ 20.9