Derivative Instruments Group 4 (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Commodity [Member] | Not Designated as Hedging Instrument [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Gains (losses) reflected in the income statement for changes in fair value of commodity contracts not designated as hedging	$ (0.3)	$ (0.9)	$ (1.1)	$ 1.0
Cash Flow Hedging [Member] | Interest charges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(5.0)	(3.9)	(10.1)	(6.8)
Cash Flow Hedging [Member] | Interest charges [Member] | Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(2.2)	(2.2)	(4.4)	(4.4)
Cash Flow Hedging [Member] | Fuel Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0	0	0	0
Cash Flow Hedging [Member] | Fuel Costs [Member] | Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0	0	0	0
Cash Flow Hedging [Member] | Income tax benefit (expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0	2.2	0.1	2.1
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	1.9	1.4	3.9	2.6
Cash Flow Hedging [Member] | Income tax benefit (expense) [Member] | Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0	0	0.1	0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0.9	0.8	1.7	1.7
Regulatory Account [Member] | Fuel Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain(Loss) Reclassified from Regulatory Account into Income (Effective Portion)	(2.0)	(1.0)	(2.7)	(2.9)
Interest Rate Contract [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0	(5.8)	0	(5.3)
Interest Rate Contract [Member] | Cash Flow Hedging [Member] | Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0	0	0	0
Commodity Contract [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0.1	(0.1)	(0.2)	(0.1)
Commodity Contract [Member] | Cash Flow Hedging [Member] | Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0.1	(0.1)	(0.2)	(0.1)
Commodity Contract [Member] | Regulatory Account [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized on Regulatory Account on Derivatives (Effective Portion)	0.3	(1.0)	(2.7)	(1.3)
Total Derivatives [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0.1	(3.7)	(0.1)	(3.3)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(3.1)	(2.5)	(6.2)	(4.2)
Total Derivatives [Member] | Cash Flow Hedging [Member] | Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0.1	(0.1)	(0.1)	(0.1)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (1.3)	$ (1.4)	$ (2.7)	$ (2.7)