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Equity Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Equity compensation expense $ 3.7 $ (0.2)
Income tax benefit $ 1.5 $ 0.0
Performance Shares [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected volatility 18.00%  
Expected dividend yield (in hundredths) 3.61%  
Risk-free interest rate 0.94%  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]    
Beginning balance (in shares) 609,010  
Granted (in shares) 225,204  
Earned (in shares) (306,953)  
Performance adjustment (in shares) 99,553  
Ending balance (in shares) 626,814  
Weighted average beginning balance (in dollars per share) $ 25.60  
Weighted average granted (in dollars per share) 31.41 $ 24.06
Weighted average earned (in dollars per share) 24.22  
Weighted average performance adjustment (in dollars per share) 24.16  
Weighted average ending balance (in dollars per share) $ 28.13  
Total unrecognized compensation expense $ 11.7  
Remaining weighted-average contractual term (in years) 1 year 11 months  
Fair value of performance shares earned and paid $ 7.4 $ 0.5
Restricted Stock [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]    
Beginning balance (in shares) 231,508  
Granted (in shares) 95,218  
Vested (in shares) (69,219)  
Ending balance (in shares) 257,507  
Weighted average beginning balance (in dollars per share) $ 24.78  
Weighted average granted (in dollars per share) 29.42 $ 26.18
Weighted average vested (in dollars per share) 22.59  
Weighted average ending balance (in dollars per share) $ 27.09  
Total unrecognized compensation expense $ 4.5  
Total fair value of shares vested $ 1.6 $ 1.9
Remaining weighted-average contractual term (in years) 1 year 9 months  
Kansas City Power and Light Company [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Equity compensation expense $ 2.5 (0.1)
Income tax benefit $ 1.0 $ 0.0