Derivative Instruments Derivative Instruments Offsetting (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivative Assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	$ 14.6	$ 8.5
Derivative Asset, Fair Value, Gross Amounts Offset	(2.7)	(0.7)
Derivative Assets, Fair Value, Net	11.9	7.8
Derivative, Collateral, Obligation to Return Securities	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative Assets Net of Gross Amounts Offset And Not Offset	11.9	7.8
Derivative Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	3.2	1.0
Derivative Liability, Fair Value, Gross Amounts Offset	(3.0)	(0.9)
Derivative Liabilities, Fair Value, Net	0.2	0.1
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative Liability, Fair Value, Net of Gross Amounts Offset And Not Offset	0.2	0.1
Kansas City Power and Light Company [Member]
Derivative Assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	5.8	1.2
Derivative Asset, Fair Value, Gross Amounts Offset	(1.7)	(0.1)
Derivative Assets, Fair Value, Net	4.1	1.1
Derivative, Collateral, Obligation to Return Securities	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative Assets Net of Gross Amounts Offset And Not Offset	4.1	1.1
Derivative Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	2.2	0.3
Derivative Liability, Fair Value, Gross Amounts Offset	(2.0)	(0.3)
Derivative Liabilities, Fair Value, Net	0.2	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative Liability, Fair Value, Net of Gross Amounts Offset And Not Offset	$ 0.2	$ 0