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Derivative Instruments (Details) (USD $)
In Millions, unless otherwise specified
1 Months Ended 12 Months Ended
May 31, 2011
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2011
Futures contracts [Member]
Cash Flow Hedging [Member]
Dec. 31, 2010
Futures contracts [Member]
Cash Flow Hedging [Member]
Dec. 31, 2011
Futures contracts [Member]
Not Designated as Hedging Instrument [Member]
Dec. 31, 2010
Futures contracts [Member]
Not Designated as Hedging Instrument [Member]
Dec. 31, 2011
Forward Contracts [Member]
Not Designated as Hedging Instrument [Member]
Dec. 31, 2010
Forward Contracts [Member]
Not Designated as Hedging Instrument [Member]
Dec. 31, 2011
Option contracts [Member]
Not Designated as Hedging Instrument [Member]
Dec. 31, 2010
Option contracts [Member]
Not Designated as Hedging Instrument [Member]
Dec. 31, 2011
Anticipated debt issuance [Member]
Forward Starting Swaps [Member]
Dec. 31, 2010
Anticipated debt issuance [Member]
Forward Starting Swaps [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Futures contracts [Member]
Cash Flow Hedging [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Futures contracts [Member]
Cash Flow Hedging [Member]
Interest Rate Risk Management [Abstract]                                  
Amount of long-term debt issued $ 350.0                                
Loss recorded in other comprehensive income on forward starting swaps settled with debt issue         26.1                        
Reclassification period for loss recorded in other comprehensive income (in years)         three years                        
Amount of loss reclassified from other comprehensive income to interest expense.         5.4                        
Commodity Risk Management [Abstract]                                  
Percentage of projected natural gas usage hedged at KCPL (in hundredths)   13.00% 56.00% 66.00%                          
Percentage of expected on-peak natural gas and natural gas equivalent purchased power price exposure that is hedged at GMO (in hundredths)   38.00% 38.00% 45.00%                          
Notional Contract Amount           2.0 4.0 23.6 59.5 97.3 202.8 0.4 0.2 0 350.0 2.0 4.0
Fair Value           $ (0.5) $ 0 $ (5.0) $ (2.5) $ 7.8 $ 8.9 $ 0 $ 0 $ 0 $ (20.8) $ (0.5) $ 0