LEASES - Supplemental Cash Flow Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 5.8
Operating cash flows from finance leases	0.7
Financing cash flows from finance leases	1.1
Right-of-use assets obtained in exchange for new operating lease liabilities	2.2
Right-of-use assets obtained in exchange for new finance lease liabilities	2.3
Westar Energy
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	3.4
Operating cash flows from finance leases	0.6
Financing cash flows from finance leases	1.1
Right-of-use assets obtained in exchange for new operating lease liabilities	0.0
Right-of-use assets obtained in exchange for new finance lease liabilities	2.3
KCP&L
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	2.4
Operating cash flows from finance leases	0.0
Financing cash flows from finance leases	0.0
Right-of-use assets obtained in exchange for new operating lease liabilities	2.2
Right-of-use assets obtained in exchange for new finance lease liabilities	$ 0.0