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Nuclear Plant (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Investments in decommissioning trust fund [Line Items]        
Cost Basis $ 154.7   $ 152.8  
Unrealized Gains 52.8   50.5  
Unrealized Losses (2.7)   (2.6)  
Fair Value $ 204.8   200.7 $ 199.0
Weighted average maturity of debt securities (in years) 7 years      
Realized gains $ 0.7 $ 1.4    
Realized losses (0.7) $ (0.6)    
Equity Securities [Member]        
Investments in decommissioning trust fund [Line Items]        
Cost Basis 90.9   89.6  
Unrealized Gains 48.8   47.9  
Unrealized Losses (2.6)   (2.1)  
Fair Value 137.1   135.4  
Debt Securities [Member]        
Investments in decommissioning trust fund [Line Items]        
Cost Basis 61.4   59.6  
Unrealized Gains 4.0   2.6  
Unrealized Losses (0.1)   (0.5)  
Fair Value 65.3   61.7  
Other Securities [Member]        
Investments in decommissioning trust fund [Line Items]        
Cost Basis 2.4   3.6  
Fair Value 2.4   3.6  
Kansas City Power and Light Company [Member]        
Investments in decommissioning trust fund [Line Items]        
Fair Value $ 204.8   $ 200.7 $ 199.0