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Derivative Instruments (Details) - USD ($)
$ in Millions
Jun. 30, 2015
Dec. 31, 2014
Futures contracts [Member] | Not Designated as Hedging Instrument [Member]    
Price Risk Derivatives [Abstract]    
Notional Contract Amount $ 34.1 $ 14.9
Fair Value (2.9) (2.4)
Forward Contracts [Member] | Not Designated as Hedging Instrument [Member]    
Price Risk Derivatives [Abstract]    
Notional Contract Amount 23.4 29.7
Fair Value 3.8 4.1
Option contracts [Member] | Not Designated as Hedging Instrument [Member]    
Price Risk Derivatives [Abstract]    
Notional Contract Amount 0.0 1.7
Fair Value 0.0 0.1
Transmission Congestion Rights [Member] | Not Designated as Hedging Instrument [Member]    
Price Risk Derivatives [Abstract]    
Notional Contract Amount 12.8 28.3
Fair Value 0.1 2.6
Kansas City Power and Light Company [Member] | Futures contracts [Member] | Not Designated as Hedging Instrument [Member]    
Price Risk Derivatives [Abstract]    
Notional Contract Amount 5.4 0.0
Fair Value 0.0 0.0
Kansas City Power and Light Company [Member] | Transmission Congestion Rights [Member] | Not Designated as Hedging Instrument [Member]    
Price Risk Derivatives [Abstract]    
Notional Contract Amount 9.5 23.6
Fair Value $ 0.1 $ 3.1
KCPL Greater Missouri Operations [Member]    
Price Risk Derivatives [Abstract]    
Percentage of expected on peak natural gas and natural gas equivalent purchased power price exposure hedged remainder of fiscal year 43.00%  
Percentage of expected on peak natural gas and natural gas equivalent purchased power price exposure hedged year two 29.00%  
Percentage of expected on peak natural gas and natural gas equivalent purchased power price exposure hedged year three 10.00%