Derivative Instruments Fair Value (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 14.6	$ 8.5
Derivative Liability, Fair Value, Gross Liability	3.2	1.0
Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	5.8	1.2
Derivative Liability, Fair Value, Gross Liability	2.2	0.3
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Balance Sheet Location [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	14.6	8.5
Derivative Liability, Fair Value, Gross Liability	3.2	1.0
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Balance Sheet Location [Member] | Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	5.8	1.2
Derivative Liability, Fair Value, Gross Liability	$ 2.2	$ 0.3