Derivative Instruments by Hedging Relationship by Income Statement Location (Details) (Regulatory Account [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Gain(Loss) Reclassified from Regulatory Account into Income	$ 1.2	$ (1.0)
Amount of Gain (Loss) Recognized on Regulatory Account on Derivatives	1.9	1.8
Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain(Loss) Reclassified from Regulatory Account into Income	0.4
Amount of Gain (Loss) Recognized on Regulatory Account on Derivatives	0.3
Fuel Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain(Loss) Reclassified from Regulatory Account into Income	0.4	(1.0)
Purchased Power Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain(Loss) Reclassified from Regulatory Account into Income	0.8
Purchased Power Costs [Member] | Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain(Loss) Reclassified from Regulatory Account into Income	0.4
Commodity Contract [Member] | Fuel Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized on Regulatory Account on Derivatives	1.1	1.8
Commodity Contract [Member] | Purchased Power Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized on Regulatory Account on Derivatives	0.8
Commodity Contract [Member] | Purchased Power Costs [Member] | Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized on Regulatory Account on Derivatives	$ 0.3