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Debt (Narrative) (Details)
$ in Millions
3 Months Ended
Feb. 05, 2016
USD ($)
Mar. 31, 2016
USD ($)
debt_covenant
fiscal_quarter
Mar. 31, 2015
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]        
Retirement of long-term debt   $ 75.4 $ 0.0  
Senior unsecured due in October 2016 - 6.5% [Member]        
Debt Instrument [Line Items]        
Debt called/redeemed $ 73.0      
Term loan outstanding   $ 57.0   $ 130.0
Debt instrument interest percentage   6.50%   6.50%
Basis points on Treasury bond yield (percent) 0.50%      
Retirement of long-term debt $ 75.4      
Revolving Credit Agreement and Standby Letters of Credit [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]        
Debt Instrument [Line Items]        
Number of financial covenants | debt_covenant   2    
Number of prior quarters included in EBITDA to interest calculation | fiscal_quarter   4    
Revolving Credit Agreement and Term Loan [Member]        
Debt Instrument [Line Items]        
Number of financial covenants | debt_covenant   2    
Number of prior quarters included in EBITDA to interest calculation | fiscal_quarter   4    
Number of prior quarters included in debt to EBITDA ratio | fiscal_quarter   4