Derivative Instruments Offsetting Assets (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 4.4	$ 8.6
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(0.5)	(1.2)
Derivative Asset	3.9	7.4
Derivative, Collateral, Obligation to Return Securities	0.0	0.0
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	3.9	7.4
Kansas City Power and Light Company [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.5	4.0
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(0.4)	(0.9)
Derivative Asset	0.1	3.1
Derivative, Collateral, Obligation to Return Securities	0.0	0.0
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	$ 0.1	$ 3.1