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Derivative Instruments and Hedging Activities (Classification within the Condensed Consolidated Statements of Results of Operations or Balance Sheets of the Gains and Losses) (Details) - Not Designated as Hedging Instrument [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Change in unrealized gain / (loss) $ 4.3 $ (5.6) $ (3.0)
Realized gain / (loss) (5.2) (10.4) (3.1)
Derivative, Gain (Loss) on Derivative, Net (0.9) (16.0) (6.1)
Commodity Contract - Heating Oil [Member]      
Change in unrealized gain / (loss) 0.0 0.4 (0.6)
Realized gain / (loss) 0.0 (0.3) (0.1)
Derivative, Gain (Loss) on Derivative, Net 0.0 0.1 (0.7)
Commodity Contract - FTR [Member]      
Change in unrealized gain / (loss) 0.3 0.3 (0.8)
Realized gain / (loss) (0.6) (0.2) 0.7
Derivative, Gain (Loss) on Derivative, Net (0.3) 0.1 (0.1)
Forward Contract Power [Member]      
Change in unrealized gain / (loss) 4.0 (6.4) (1.5)
Realized gain / (loss) (7.2) (9.8) (3.6)
Derivative, Gain (Loss) on Derivative, Net (3.2) (16.2) (5.1)
Natural Gas [Member]      
Change in unrealized gain / (loss) 0.0 0.1 (0.1)
Realized gain / (loss) 2.6 (0.1) (0.1)
Derivative, Gain (Loss) on Derivative, Net 2.6 0.0 (0.2)
THE DAYTON POWER AND LIGHT COMPANY [Member]      
Change in unrealized gain / (loss) 4.2 (5.5) (3.0)
Realized gain / (loss) (5.9) (10.5) (2.5)
Derivative, Gain (Loss) on Derivative, Net (1.7) (16.0) (5.5)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]      
Change in unrealized gain / (loss) 0.0 0.4 (0.6)
Realized gain / (loss) 0.0 (0.3) (0.1)
Derivative, Gain (Loss) on Derivative, Net 0.0 0.1 (0.7)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]      
Change in unrealized gain / (loss) 0.3 0.3 (0.8)
Realized gain / (loss) (0.6) (0.2) 0.7
Derivative, Gain (Loss) on Derivative, Net (0.3) 0.1 (0.1)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]      
Change in unrealized gain / (loss) 3.9 (6.3) (1.5)
Realized gain / (loss) (7.9) (9.9) (3.0)
Derivative, Gain (Loss) on Derivative, Net (4.0) (16.2) (4.5)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]      
Change in unrealized gain / (loss) 0.0 0.1 (0.1)
Realized gain / (loss) 2.6 (0.1) (0.1)
Derivative, Gain (Loss) on Derivative, Net 2.6 0.0 (0.2)
Fuel [Member]      
Derivative, Gain (Loss) on Derivative, Net     (0.6)
Fuel [Member] | Commodity Contract - Heating Oil [Member]      
Derivative, Gain (Loss) on Derivative, Net     (0.6)
Fuel [Member] | Commodity Contract - FTR [Member]      
Derivative, Gain (Loss) on Derivative, Net     0.0
Fuel [Member] | Forward Contract Power [Member]      
Derivative, Gain (Loss) on Derivative, Net     0.0
Fuel [Member] | Natural Gas [Member]      
Derivative, Gain (Loss) on Derivative, Net     0.0
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Derivative, Gain (Loss) on Derivative, Net     (0.6)
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]      
Derivative, Gain (Loss) on Derivative, Net     (0.6)
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]      
Derivative, Gain (Loss) on Derivative, Net     0.0
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]      
Derivative, Gain (Loss) on Derivative, Net     0.0
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]      
Derivative, Gain (Loss) on Derivative, Net     0.0
Purchased Power [Member]      
Derivative, Gain (Loss) on Derivative, Net 16.4 (43.5) (5.4)
Purchased Power [Member] | Commodity Contract - Heating Oil [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.0 0.0
Purchased Power [Member] | Commodity Contract - FTR [Member]      
Derivative, Gain (Loss) on Derivative, Net (0.3) 0.1 (0.1)
Purchased Power [Member] | Forward Contract Power [Member]      
Derivative, Gain (Loss) on Derivative, Net 14.1 (43.6) (5.1)
Purchased Power [Member] | Natural Gas [Member]      
Derivative, Gain (Loss) on Derivative, Net 2.6 0.0 (0.2)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Derivative, Gain (Loss) on Derivative, Net 16.4 (43.5) (5.5)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.0 0.0
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]      
Derivative, Gain (Loss) on Derivative, Net (0.3) 0.1 (0.1)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]      
Derivative, Gain (Loss) on Derivative, Net 14.1 (43.6) (5.2)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]      
Derivative, Gain (Loss) on Derivative, Net 2.6 0.0 (0.2)
Regulatory Asset [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.1 (0.1)
Regulatory Asset [Member] | Commodity Contract - Heating Oil [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.1 (0.1)
Regulatory Asset [Member] | Commodity Contract - FTR [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.0 0.0
Regulatory Asset [Member] | Forward Contract Power [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.0 0.0
Regulatory Asset [Member] | Natural Gas [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.0 0.0
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.1 (0.1)
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.1 (0.1)
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.0 0.0
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]      
Derivative, Gain (Loss) on Derivative, Net 0.0 0.0 0.0
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]      
Derivative, Gain (Loss) on Derivative, Net $ 0.0 $ 0.0 $ 0.0