Pension Plans and Other Employee Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Amounts recognized in the consolidated balance sheets [Abstract]
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	$ (2.8)		$ (2.8)		$ (2.8)
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	(549.9)		(549.9)		(557.5)
Other Changes In Plan Assets And Benefit Obligations Recognized In Other Comprehensive Income Regulatory Assets/Liabilities [Abstract]
Amortization of net losses included in net periodic benefit costs	(0.3)	(0.1)	(0.3)	(0.2)
Kansas City Power and Light Company [Member]
Amounts recognized in the consolidated balance sheets [Abstract]
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	(1.4)		(1.4)		(1.4)
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	(527.1)		(527.1)		(534.5)
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Contributions by Employer			32.4
Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year	25.0		25.0
Components of net periodic benefit costs [Abstract]
Service cost	10.5	8.8	21.0	17.7
Interest cost	11.8	12.3	23.6	24.5
Defined Benefit Plan, Expected Return on Plan Assets	(11.8)	(10.7)	(23.6)	(21.4)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.5	1.1	1.0	2.2
Defined Benefit Plan, Amortization of Gains (Losses)	13.7	11.2	27.4	22.3
Defined Benefit Plan, Amortization of Transition Obligations (Assets)	0	0	0	0
Net periodic benefit costs before regulatory adjustment	24.7	22.7	49.4	45.3
Defined benefit plan, regulatory adjustment	(2.8)	(3.8)	(6.4)	(7.7)
Defined Benefit Plan, Net Periodic Benefit Cost	21.9	18.9	43.0	37.6
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year	18.7		18.7
Components of net periodic benefit costs [Abstract]
Service cost	1.1	0.8	2.2	1.6
Interest cost	1.9	2.0	3.8	3.9
Defined Benefit Plan, Expected Return on Plan Assets	(0.5)	(0.4)	(1.0)	(0.9)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	1.8	1.8	3.6	3.6
Defined Benefit Plan, Amortization of Gains (Losses)	0.4	0	0.9	0
Defined Benefit Plan, Amortization of Transition Obligations (Assets)	0.1	0.2	0.1	0.5
Net periodic benefit costs before regulatory adjustment	4.8	4.4	9.6	8.7
Defined benefit plan, regulatory adjustment	(0.6)	0.3	(1.1)	0.7
Defined Benefit Plan, Net Periodic Benefit Cost	$ 4.2	$ 4.7	$ 8.5	$ 9.4