Derivative Instruments Fair Value (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivative [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral	$ 6.5	$ 7.8
Derivative liability, fair value, amount not offset against collateral	3.0	5.5
Cash Flow Hedging [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	(35.4)	(47.9)
Cash Flow Hedging [Member] | Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	(25.8)	(31.4)
Cash Flow Hedging [Member] | Current assets [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	10.6	11.3
Cash Flow Hedging [Member] | Current assets [Member] | Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	10.6	11.3
Cash Flow Hedging [Member] | Current liabilities [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	(68.4)	(89.5)
Cash Flow Hedging [Member] | Current liabilities [Member] | Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	(52.8)	(62.5)
Cash Flow Hedging [Member] | Noncurrent liabilities [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	(0.1)	(0.2)
Cash Flow Hedging [Member] | Noncurrent liabilities [Member] | Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	(0.1)	(0.2)
Cash Flow Hedging [Member] | Deferred income taxes [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	22.5	30.5
Cash Flow Hedging [Member] | Deferred income taxes [Member] | Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	16.5	20.0
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Derivative [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral	0	0
Derivative liability, fair value, amount not offset against collateral	0.2	0.5
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Derivative [Member] | Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral	0	0
Derivative liability, fair value, amount not offset against collateral	0.2	0.5
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Derivative [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral	6.5	7.8
Derivative liability, fair value, amount not offset against collateral	$ 2.8	$ 5.0