Derivative Instruments, Group 2 (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Cash Flow Hedging [Member] | Derivative Instrument, Current and Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	$ (38.4)	$ (47.9)
Cash Flow Hedging [Member] | Derivative Instrument, Current and Noncurrent [Member] | Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	(27.1)	(31.4)
Cash Flow Hedging [Member] | Current assets [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	10.8	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	10.8	11.3
Cash Flow Hedging [Member] | Current liabilities [Member]
Derivatives, Fair Value [Line Items]
Recorded amounts included in accumulated other comprehensive income	(73.6)	(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	(55.1)	(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	24.5	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	17.3	20.0
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Derivative Instrument, Current and Noncurrent [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 Instrument, Current and Noncurrent [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 Instrument, Current and Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral	7.2	7.8
Derivative liability, fair value, amount not offset against collateral	2.3	5.0
Total Derivatives [Member] | Derivative Instrument, Current and Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral	7.2	7.8
Derivative liability, fair value, amount not offset against collateral	$ 2.5	$ 5.5