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Nuclear Plant (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Investments in decommissioning trust fund [Line Items]      
Cost Basis $ 152.8 $ 145.4  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 50.5 54.4  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax (2.6) (0.8)  
Fair Value, Nuclear decommissioning trust fund $ 200.7 199.0 $ 183.9
Weighted average maturity of debt securities (in years) 8 years    
Realized gains on investments in decommissioning trust fund $ 5.3 1.4 2.4
Realized losses on investments in decommissioning trust fund (4.6) (1.0) (0.7)
Equity Securities [Member]      
Investments in decommissioning trust fund [Line Items]      
Cost Basis 89.6 87.2  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax 47.9 50.6  
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax (2.1) (0.7)  
Fair Value, Nuclear decommissioning trust fund 135.4 137.1  
Debt Securities [Member]      
Investments in decommissioning trust fund [Line Items]      
Cost Basis 59.6 55.4  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 2.6 3.8  
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax (0.5) (0.1)  
Fair Value, Nuclear decommissioning trust fund 61.7 59.1  
Other Securities [Member]      
Investments in decommissioning trust fund [Line Items]      
Cost Basis 3.6 2.8  
Fair Value, Nuclear decommissioning trust fund 3.6 2.8  
Kansas City Power and Light Company [Member]      
Investments in decommissioning trust fund [Line Items]      
Fair Value, Nuclear decommissioning trust fund $ 200.7 $ 199.0 $ 183.9