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Fair Value (Fair Value and Cost of Non-Derivative Instruments) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Realized gains on investments sold to facilitate the distribution of benefits, after tax $ 0.5  
Unrealized Gains in AOCI, Net of Tax 0.8  
Unrealized Gains and Immaterial Unrealized Losses in AOCI, Before Tax   $ 1.0
THE DAYTON POWER AND LIGHT COMPANY [Member]    
Realized gains on investments sold to facilitate the distribution of benefits, after tax 0.5  
Unrealized Gains in AOCI, Net of Tax 0.9  
Unrealized Gains and Immaterial Unrealized Losses in AOCI, Before Tax   1.1
Cost [Member]    
Total Assets 7.4 7.6
Cost [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Assets 7.4 7.6
Fair Value [Member]    
Total Master Trust Assets, Fair Value 8.7 8.7
Total Assets 8.7 8.7
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Fair Value 8.7 8.7
Total Assets 8.7 8.7
Money Market Funds [Member] | Cost [Member]    
Total Master Trust Assets, Cost 0.3 0.4
Money Market Funds [Member] | Cost [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Cost 0.3 0.4
Money Market Funds [Member] | Fair Value [Member]    
Total Master Trust Assets, Fair Value 0.3 0.4
Money Market Funds [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Fair Value 0.3 0.4
Equity Securities [Member] | Cost [Member]    
Total Master Trust Assets, Cost 2.6 2.4
Equity Securities [Member] | Cost [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Cost 2.6 2.4
Equity Securities [Member] | Fair Value [Member]    
Total Master Trust Assets, Fair Value 3.9 3.4
Equity Securities [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Fair Value 3.9 3.4
Debt Securities [Member] | Cost [Member]    
Total Master Trust Assets, Cost 4.3 4.4
Debt Securities [Member] | Cost [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Cost 4.3 4.4
Debt Securities [Member] | Fair Value [Member]    
Total Master Trust Assets, Fair Value 4.3 4.4
Debt Securities [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Fair Value 4.3 4.4
Hedge Funds [Member] | Cost [Member]    
Total Master Trust Assets, Cost 0.1 0.0
Hedge Funds [Member] | Cost [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Cost 0.1 0.0
Hedge Funds [Member] | Fair Value [Member]    
Total Master Trust Assets, Fair Value 0.1 0.1
Hedge Funds [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Fair Value 0.1 0.1
Real Estate Funds [Member] | Cost [Member]    
Total Master Trust Assets, Cost 0.0 0.3
Real Estate Funds [Member] | Cost [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Cost 0.0 0.3
Real Estate Funds [Member] | Fair Value [Member]    
Total Master Trust Assets, Fair Value 0.0 0.3
Real Estate Funds [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Fair Value 0.0 0.3
Tangible Assets [Member] | Cost [Member]    
Total Master Trust Assets, Cost 0.1 0.1
Tangible Assets [Member] | Cost [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Cost 0.1 0.1
Tangible Assets [Member] | Fair Value [Member]    
Total Master Trust Assets, Fair Value 0.1 0.1
Tangible Assets [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Total Master Trust Assets, Fair Value 0.1 0.1
Debt [Member] | Cost [Member]    
Debt, Cost 1,851.6 1,858.0
Debt [Member] | Cost [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Debt, Cost 748.3 749.0
Debt [Member] | Fair Value [Member]    
Debt, Fair Value 1,940.7 1,907.7
Debt [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Debt, Fair Value $ 762.3 $ 763.5