Derivative Instruments Offsetting Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Offsetting Liabilities [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ 3.4	$ 4.2
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(3.3)	(3.5)
Derivative Liability	0.1	0.7
Derivative, Collateral, Right to Reclaim Securities	0.0	0.0
Derivative, Collateral, Right to Reclaim Cash	0.0	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	0.1	0.7
Cash collateral posted with third parties as part of netting agreements	2.8	2.3
Kansas City Power and Light Company [Member]
Offsetting Liabilities [Line Items]
Derivative Liability, Fair Value, Gross Liability	0.4	0.9
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(0.4)	(0.9)
Derivative Liability	0.0	0.0
Derivative, Collateral, Right to Reclaim Securities	0.0	0.0
Derivative, Collateral, Right to Reclaim Cash	0.0	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ 0.0	$ 0.0