Pension Plans and Other Employee Benefits (Details) (USD $) In Millions, unless otherwise specified					3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Kansas City Power and Light Company [Member]	Dec. 31, 2013 Kansas City Power and Light Company [Member]	Mar. 31, 2014 Pension Plans, Defined Benefit [Member]	Mar. 31, 2013 Pension Plans, Defined Benefit [Member]	Mar. 31, 2014 Other Postretirement Benefit Plans, Defined Benefit [Member]	Mar. 31, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Contributions by Employer					$ 3.3
Amounts recognized in the consolidated balance sheets [Abstract]
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	(3.2)	(3.2)	(1.9)	(1.9)
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	(367.5)	(360.5)	(347.0)	(339.9)
Components of net periodic benefit costs [Abstract]
Defined Benefit Plan, Service Cost					9.1	10.5	0.9	1.1
Defined Benefit Plan, Interest Cost					12.7	11.8	2.0	1.9
Defined Benefit Plan, Expected Return on Plan Assets					(12.7)	(11.8)	(0.7)	(0.5)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)					0.2	0.5	0.8	1.8
Defined Benefit Plan, Amortization of Gains (Losses)					12.4	13.7	0	0.5
Net periodic benefit costs before regulatory adjustment					21.7	24.7	3.0	4.8
Defined benefit plan, regulatory adjustment					(0.4)	(3.6)	1.1	(0.5)
Defined Benefit Plan, Net Periodic Benefit Cost					21.3	21.1	4.1	4.3
Contribute to plans in next year					$ 58.9		$ 11.3