Derivative Instruments Offsetting Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Offsetting Liabilities [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ 80.8	$ 6.4
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(2.0)	(5.9)
Derivative Liability	78.8	0.5
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	78.8	0.5
Cash collateral posted with counterparties	0.1	5.7
Kansas City Power and Light Company [Member]
Offsetting Liabilities [Line Items]
Derivative Liability, Fair Value, Gross Liability	0.7	0.7
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(0.7)	(0.3)
Derivative Liability	0.0	0.4
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	$ 0.4