PENSION PLANS AND POST-RETIREMENT BENEFITS (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits
Components of net periodic benefit costs [Abstract]
Service cost	$ 19.1	$ 8.0
Interest cost	27.5	12.7
Expected return on plan assets	(27.1)	(14.0)
Prior service cost	0.5	0.2
Recognized net actuarial (gain)/loss	6.9	8.2
Net periodic benefit costs before regulatory adjustment and intercompany allocations	26.9	15.1
Regulatory adjustment	(0.1)	2.8
Intercompany allocations	0.0	0.0
Net periodic benefit costs	26.8	17.9
Contributions by employer	12.3
Contributions by employer, remainder of fiscal year	103.2
Pension Benefits | Westar Energy
Components of net periodic benefit costs [Abstract]
Service cost	7.3	8.0
Interest cost	13.4	12.7
Expected return on plan assets	(13.7)	(14.0)
Prior service cost	0.4	0.2
Recognized net actuarial (gain)/loss	6.4	8.2
Net periodic benefit costs before regulatory adjustment and intercompany allocations	13.8	15.1
Regulatory adjustment	0.5	2.8
Intercompany allocations	0.0	0.0
Net periodic benefit costs	14.3	17.9
Contributions by employer	7.1
Contributions by employer, remainder of fiscal year	29.9
Pension Benefits | KCP&L
Components of net periodic benefit costs [Abstract]
Service cost	11.8	12.2
Interest cost	14.1	12.7
Expected return on plan assets	(12.3)	(13.9)
Prior service cost	0.2	0.2
Recognized net actuarial (gain)/loss	12.2	11.4
Net periodic benefit costs before regulatory adjustment and intercompany allocations	26.0	22.6
Regulatory adjustment	(0.6)	0.4
Intercompany allocations	(6.9)	(5.5)
Net periodic benefit costs	18.5	17.5
Contributions by employer	5.2
Contributions by employer, remainder of fiscal year	73.3
Post-Retirement Benefits
Components of net periodic benefit costs [Abstract]
Service cost	0.6	0.3
Interest cost	2.6	1.2
Expected return on plan assets	(2.4)	(1.7)
Prior service cost	0.1	0.1
Recognized net actuarial (gain)/loss	(0.3)	(0.1)
Net periodic benefit costs before regulatory adjustment and intercompany allocations	0.6	(0.2)
Regulatory adjustment	(0.7)	(0.4)
Intercompany allocations	0.0	0.0
Net periodic benefit costs	(0.1)	(0.6)
Contributions by employer	2.8
Post-Retirement Benefits | Westar Energy
Components of net periodic benefit costs [Abstract]
Service cost	0.3	0.3
Interest cost	1.4	1.2
Expected return on plan assets	(1.7)	(1.7)
Prior service cost	0.1	0.1
Recognized net actuarial (gain)/loss	(0.1)	(0.1)
Net periodic benefit costs before regulatory adjustment and intercompany allocations	0.0	(0.2)
Regulatory adjustment	(0.8)	(0.4)
Intercompany allocations	0.0	0.0
Net periodic benefit costs	(0.8)	(0.6)
Contributions by employer	0.7
Post-Retirement Benefits | KCP&L
Components of net periodic benefit costs [Abstract]
Service cost	0.3	0.5
Interest cost	1.2	1.2
Expected return on plan assets	(0.7)	(0.7)
Prior service cost	0.0	0.0
Recognized net actuarial (gain)/loss	(0.4)	0.0
Net periodic benefit costs before regulatory adjustment and intercompany allocations	0.4	1.0
Regulatory adjustment	0.1	(0.1)
Intercompany allocations	(0.1)	(0.3)
Net periodic benefit costs	0.4	$ 0.6
Contributions by employer	$ 2.1
Wolf Creek
Defined Benefit Plan Disclosure [Line Items]
Jointly owned utility plant, proportionate ownership share	94.00%
Wolf Creek | Westar Energy
Defined Benefit Plan Disclosure [Line Items]
Jointly owned utility plant, proportionate ownership share	47.00%
Wolf Creek | KCP&L
Defined Benefit Plan Disclosure [Line Items]
Jointly owned utility plant, proportionate ownership share	47.00%