Pension Plans and Other Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Contributions by Employer			$ 23.6
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year			52.4
Components of net periodic benefit costs [Abstract]
Service cost	$ 10.5	$ 11.4	31.5	$ 34.0
Interest cost	13.2	12.5	39.7	37.7
Expected return on plan assets	(12.3)	(13.0)	(36.9)	(38.8)
Prior service cost	0.2	0.2	0.5	0.6
Recognized net actuarial (gain)/loss	13.0	12.9	38.9	38.5
Transition obligation	0.0	0.0	0.0	0.0
Net periodic benefit costs before regulatory adjustment	24.6	24.0	73.7	72.0
Regulatory adjustment	(1.1)	(3.5)	(3.1)	(9.3)
Net periodic benefit costs	23.5	20.5	70.6	62.7
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year			5.1
Components of net periodic benefit costs [Abstract]
Service cost	0.7	0.8	2.0	2.5
Interest cost	1.5	1.7	4.6	5.1
Expected return on plan assets	(0.8)	(0.7)	(2.3)	(2.2)
Prior service cost	0.3	0.7	0.9	2.3
Recognized net actuarial (gain)/loss	(0.3)	0.1	(1.1)	0.2
Transition obligation	0.0	0.1	0.0	0.1
Net periodic benefit costs before regulatory adjustment	1.4	2.7	4.1	8.0
Regulatory adjustment	1.4	1.4	4.4	4.2
Net periodic benefit costs	$ 2.8	$ 4.1	$ 8.5	$ 12.2
Wolf Creek [Member] | Kansas City Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	47.00%		47.00%		47.00%