Derivative Instruments by Hedging Relationship by Income Statement Location (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 0	$ 0.1	$ 0	$ (0.1)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(2.9)	(3.1)	(6.1)	(6.2)
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	0.1	0	(0.1)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.3)	(1.3)	(2.8)	(2.7)
Cash Flow Hedging [Member] | Interest charges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(4.7)	(5.0)	(9.7)	(10.1)
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	(0.2)	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	(0.2)	0
Cash Flow Hedging [Member] | Income tax benefit (expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	1.8	1.9	3.8	3.9
Cash Flow Hedging [Member] | Income tax benefit (expense) [Member] | Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0.9	0.9	1.8	1.7
Regulatory Account [Member] | Fuel Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain(Loss) Reclassified from Regulatory Account into Income (Effective Portion)	(0.2)	(2.0)	(1.2)	(2.7)
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	0
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	(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	(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)	0.3	0.9	(2.7)
Income tax benefit (expense) [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	0.1
Income tax benefit (expense) [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	$ 0.1