Derivative Instruments Group 4 (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
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)	$ (5.1)	$ (15.1)	$ (11.9)
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)	(2.1)	(6.6)	(6.5)
Cash Flow Hedging [Member] | Fuel Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(0.5)	(0.1)	(0.5)	(0.1)
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)	(0.5)	(0.1)	(0.5)	(0.1)
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)	(0.1)	0.1	0	2.2
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	2.1	2.1	6.0	4.7
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)	(0.1)	0.1	0	0.1
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	1.0	0.9	2.7	2.6
Regulatory Account [Member] | Fuel Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain(Loss) Reclassified from Regulatory Account into Income (Effective Portion)	(2.8)	(0.6)	(5.5)	(3.5)
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	0	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)	(0.1)	(0.2)
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)	(0.1)	(0.2)
Commodity Contract [Member] | Regulatory Account [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized on Regulatory Account on Derivatives (Effective Portion)	0.9	(2.2)	(1.8)	(3.5)
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	0	(0.1)	(3.3)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(3.4)	(3.1)	(9.6)	(7.3)
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	0	(0.1)	(0.1)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (1.7)	$ (1.3)	$ (4.4)	$ (4.0)