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Fair Value Measurements (Significant unobservalbe inputs, nonrecurring) (Details) - USD ($)
$ in Millions
 3 Months Ended
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2014
East Bend Station [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value     $ 2.7
Income Approach Valuation Technique [Member] | Minimum [Member] | Conesville [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (19.00%)    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent (54.00%)    
Income Approach Valuation Technique [Member] | Minimum [Member] | Stuart [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (12.00%)    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent (61.00%)    
Income Approach Valuation Technique [Member] | Minimum [Member] | Killen [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (14.00%) (11.00%)  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent (57.00%) (50.00%)  
Income Approach Valuation Technique [Member] | Minimum [Member] | Peaking Generating Facilities [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent)   (22.00%)  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent   (29.00%)  
Income Approach Valuation Technique [Member] | Minimum [Member] | Hutchings Peakers [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (20.00%)    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent (40.00%)    
Income Approach Valuation Technique [Member] | Minimum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Conesville [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (19.00%)    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent (54.00%)    
Income Approach Valuation Technique [Member] | Minimum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (12.00%) (9.00%)  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent (61.00%) (29.00%)  
Income Approach Valuation Technique [Member] | Minimum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Killen [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (14.00%) (11.00%)  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent (57.00%) (50.00%)  
Income Approach Valuation Technique [Member] | Minimum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent)   (14.00%)  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent   (46.00%)  
Income Approach Valuation Technique [Member] | Minimum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Hutchings Peakers [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (20.00%)    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent (40.00%)    
Income Approach Valuation Technique [Member] | Maximum [Member] | Conesville [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) 11.00%    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 99.00%    
Income Approach Valuation Technique [Member] | Maximum [Member] | Stuart [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) 1.00%    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 75.00%    
Income Approach Valuation Technique [Member] | Maximum [Member] | Killen [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) 3.00% 13.00%  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 42.00% 67.00%  
Income Approach Valuation Technique [Member] | Maximum [Member] | Peaking Generating Facilities [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent)   17.00%  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent   24.00%  
Income Approach Valuation Technique [Member] | Maximum [Member] | Hutchings Peakers [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) 26.00%    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 63.00%    
Income Approach Valuation Technique [Member] | Maximum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Conesville [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) 11.00%    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 99.00%    
Income Approach Valuation Technique [Member] | Maximum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) 1.00% 10.00%  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 75.00% 52.00%  
Income Approach Valuation Technique [Member] | Maximum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Killen [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) 3.00% 13.00%  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 42.00% 67.00%  
Income Approach Valuation Technique [Member] | Maximum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent)   13.00%  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent   80.00%  
Income Approach Valuation Technique [Member] | Maximum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Hutchings Peakers [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) 26.00%    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 63.00%    
Income Approach Valuation Technique [Member] | Weighted Average [Member] | Conesville [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) 1.00%    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 20.00%    
Income Approach Valuation Technique [Member] | Weighted Average [Member] | Stuart [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (5.00%)    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 8.00%    
Fair Value Inputs, Discount Rate 10.00%    
Income Approach Valuation Technique [Member] | Weighted Average [Member] | Killen [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (8.00%) 2.00%  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent (16.00%) 6.00%  
Fair Value Inputs, Discount Rate 10.00% 11.00%  
Income Approach Valuation Technique [Member] | Weighted Average [Member] | Peaking Generating Facilities [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent)   (3.00%)  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent   (4.00%)  
Fair Value Inputs, Discount Rate   7.00%  
Income Approach Valuation Technique [Member] | Weighted Average [Member] | Hutchings Peakers [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (1.00%)    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 12.00%    
Fair Value Inputs, Discount Rate 7.00%    
Income Approach Valuation Technique [Member] | Weighted Average [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Conesville [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) 1.00%    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 20.00%    
Income Approach Valuation Technique [Member] | Weighted Average [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (5.00%) 2.00%  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 8.00% 5.00%  
Fair Value Inputs, Discount Rate 10.00% 9.00%  
Income Approach Valuation Technique [Member] | Weighted Average [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Killen [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (8.00%) 2.00%  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent (16.00%) 6.00%  
Fair Value Inputs, Discount Rate 10.00% 11.00%  
Income Approach Valuation Technique [Member] | Weighted Average [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent)   1.00%  
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent   4.00%  
Fair Value Inputs, Discount Rate   9.00%  
Income Approach Valuation Technique [Member] | Weighted Average [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Hutchings Peakers [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Annual revenue growth (percent) (1.00%)    
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent 12.00%    
Fair Value Inputs, Discount Rate 7.00%    
Fair Value, Inputs, Level 3 [Member] | Conesville [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value $ 1.1    
Fair Value, Inputs, Level 3 [Member] | Miami Fort [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value 36.5    
Fair Value, Inputs, Level 3 [Member] | Stuart [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value 57.4    
Fair Value, Inputs, Level 3 [Member] | Killen [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value 42.8 $ 84.3  
Fair Value, Inputs, Level 3 [Member] | Zimmer [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value 23.7    
Fair Value, Inputs, Level 3 [Member] | East Bend Station [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value     $ 2.7
Fair Value, Inputs, Level 3 [Member] | Peaking Generating Facilities [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value   5.2  
Fair Value, Inputs, Level 3 [Member] | Hutchings Peakers [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value 1.6    
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Conesville [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value 1.1    
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Miami Fort [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value 36.5    
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value 57.4 164.4  
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Killen [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value 42.8 84.3  
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value 23.7 $ 111.0  
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Hutchings Peakers [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Long-lived assets held and used, fair value $ 1.6