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Nuclear Plant (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2013
Dec. 31, 2012
Investments in decommissioning trust fund [Line Items]            
Cost Basis $ 143.7   $ 143.7   $ 138.1  
Gross Unrealized Gains     50.1   47.1  
Gross Unrealized Losses     (0.6)   (1.3)  
Fair Value, Nuclear decommissioning trust fund 193.2   193.2   183.9 154.7
Weighted average maturity of debt securities (in years) 7   7      
Realized gains on investments in decommissioning trust fund 0.3 0.4 1.0 2.3    
Realized losses on investments in decommissioning trust fund (0.1) (0.3) (0.7) (0.7)    
Equity Securities [Member]
           
Investments in decommissioning trust fund [Line Items]            
Cost Basis 86.3   86.3   83.7  
Gross Unrealized Gains     46.9   44.6  
Gross Unrealized Losses     (0.4)   (0.6)  
Fair Value, Nuclear decommissioning trust fund 132.8   132.8   127.7  
Debt Securities [Member]
           
Investments in decommissioning trust fund [Line Items]            
Cost Basis 54.3   54.3   51.0  
Gross Unrealized Gains     3.2   2.5  
Gross Unrealized Losses     (0.2)   (0.7)  
Fair Value, Nuclear decommissioning trust fund 57.3   57.3   52.8  
Other Securities [Member]
           
Investments in decommissioning trust fund [Line Items]            
Cost Basis 3.1   3.1   3.4  
Gross Unrealized Gains     0   0  
Gross Unrealized Losses     0   0  
Fair Value, Nuclear decommissioning trust fund 3.1   3.1   3.4  
Kansas City Power and Light Company [Member]
           
Investments in decommissioning trust fund [Line Items]            
Fair Value, Nuclear decommissioning trust fund $ 193.2   $ 193.2   $ 183.9 $ 154.7