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Benefit Plans (Fair Value Measurements for Pension Plan Assets) (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities $ 60.3 $ 65.4  
Total Debt Securities 232.1 255.2  
Cash   1.6  
Total Other Investments 53.0 49.5  
Total Pension Plan Assets 345.4 371.7  
Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 60.3 65.4  
Total Debt Securities 232.1 255.2  
Cash   1.6  
Total Other Investments 53.0 49.5  
Total Pension Plan Assets 345.4 371.7 $ 349.1
Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Pension Plan Assets 345.4 371.7 $ 349.1
Level 1 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 60.3 65.4  
Total Debt Securities 232.1 255.2  
Cash   1.6  
Total Pension Plan Assets 292.4 322.2  
Level 1 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 60.3 65.4  
Total Debt Securities 232.1 255.2  
Cash   1.6  
Total Pension Plan Assets 292.4 322.2  
Level 2 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Other Investments 53.0 49.5  
Total Pension Plan Assets 53.0 49.5  
Level 2 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Other Investments 53.0 49.5  
Total Pension Plan Assets 53.0 49.5  
Small/Mid Cap Equity [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 9.2 10.6  
Small/Mid Cap Equity [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 9.2 10.6  
Small/Mid Cap Equity [Member] | Level 1 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 9.2 10.6  
Small/Mid Cap Equity [Member] | Level 1 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 9.2 10.6  
Large Cap Equity [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 20.2 22.2  
Large Cap Equity [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 20.2 22.2  
Large Cap Equity [Member] | Level 1 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 20.2 22.2  
Large Cap Equity [Member] | Level 1 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 20.2 22.2  
International Equity [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 18.2 18.2  
International Equity [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 18.2 18.2  
International Equity [Member] | Level 1 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 18.2 18.2  
International Equity [Member] | Level 1 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 18.2 18.2  
Emerging markets equity [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 2.7 2.8  
Emerging markets equity [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 2.7 2.8  
Emerging markets equity [Member] | Level 1 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 2.7 2.8  
Emerging markets equity [Member] | Level 1 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 2.7 2.8  
SIIT dynamic equity [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 10.0 11.6  
SIIT dynamic equity [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 10.0 11.6  
SIIT dynamic equity [Member] | Level 1 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 10.0 11.6  
SIIT dynamic equity [Member] | Level 1 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Equity Securities 10.0 11.6  
Emerging Markets Debt [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 6.3 6.0  
Emerging Markets Debt [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 6.3 6.0  
Emerging Markets Debt [Member] | Level 1 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 6.3 6.0  
Emerging Markets Debt [Member] | Level 1 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 6.3 6.0  
High Yield Bond [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 6.3 6.5  
High Yield Bond [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 6.3 6.5  
High Yield Bond [Member] | Level 1 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 6.3 6.5  
High Yield Bond [Member] | Level 1 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 6.3 6.5  
Long Duration Fund [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 219.5 242.7  
Long Duration Fund [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 219.5 242.7  
Long Duration Fund [Member] | Level 1 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 219.5 242.7  
Long Duration Fund [Member] | Level 1 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Debt Securities 219.5 242.7  
Core Property Collective Fund [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Other Investments 30.2 26.3  
Core Property Collective Fund [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Other Investments 30.2 26.3  
Core Property Collective Fund [Member] | Level 2 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Other Investments 30.2 26.3  
Core Property Collective Fund [Member] | Level 2 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Other Investments 30.2 26.3  
Common Collective Fund [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Other Investments 22.8 23.2  
Common Collective Fund [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Other Investments 22.8 23.2  
Common Collective Fund [Member] | Level 2 [Member] | Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Other Investments 22.8 23.2  
Common Collective Fund [Member] | Level 2 [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total Other Investments $ 22.8 $ 23.2