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Fair Value Measurements Fair Value Narrative (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Jun. 30, 2016
USD ($)
Fair value of financial instruments [Abstract]              
Derivatives, Interest Rate Swaps, Finalized Settlement Value $ 140.6   $ 140.6        
Great Plains Energy [Member]              
Fair value of financial instruments [Abstract]              
Notional Contract Amount             $ 4,400.0
Derivatives, Interest Rate Swaps, Finalized Settlement Value         $ 140.6    
KCPL Greater Missouri Operations [Member]              
SERP rabbi trust [Abstract]              
Alternative Investments, Fair Value Disclosure 15.0   15.0     $ 16.0  
Carrying (Reported) Amount, Fair Value Disclosure [Member]              
Fair value of financial instruments [Abstract]              
Long-term Debt 3,700.0   3,700.0     3,800.0  
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Kansas City Power and Light Company [Member]              
Fair value of financial instruments [Abstract]              
Long-term Debt 2,600.0   2,600.0     2,600.0  
GPE 4.3 Billion Senior Notes [Member] | Great Plains Energy [Member]              
Fair value of financial instruments [Abstract]              
Long-term Debt, Gross         $ 4,300.0    
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]              
Fair value of financial instruments [Abstract]              
Long-term debt fair value 3,900.0   3,900.0     4,000.0  
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member] | Kansas City Power and Light Company [Member]              
Fair value of financial instruments [Abstract]              
Long-term debt fair value $ 2,800.0   $ 2,800.0     $ 2,700.0  
Interest Rate Contract [Member]              
Fair value of financial instruments [Abstract]              
Fair Value Inputs, Contingency Factor, Interest Rate Derivative 0.45   0.45     0.35  
Derivative Asset, Fair Value, Gross Asset $ 77.4   $ 77.4     $ 79.3  
Interest Rate Contract [Member] | Interest Expense [Member]              
Fair value of financial instruments [Abstract]              
Derivative, Gain (Loss) on Derivative, Net $ 28.2 $ (1.8) $ (1.9) $ (78.8)