Derivative Instruments Gain and Losses (Details) (Not Designated as Hedging Instrument [Member], Commodity Contract [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 Purchased Power Costs [Member]	Mar. 31, 2014 Purchased Power Costs [Member] Kansas City Power and Light Company [Member]	Mar. 31, 2013 Fuel Costs [Member]	Mar. 31, 2013 Fuel Costs [Member] Kansas City Power and Light Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) reflected in the income statement for changes in fair value of commodity contracts not designated as hedging	$ 0.4	$ 0.4	$ (1.0)	$ (1.0)