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Income Taxes (Effective and Statutory Rate Reconciliation) (Details) - THE DAYTON POWER AND LIGHT COMPANY [Member]
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Entity Information [Line Items]              
Effective Income Tax Rate Reconciliation, Including Discontinued Operations, Percent (19.10%)   4.70%        
Estimated Annual Effective Income Tax Rate, Continuing Operations     4.50% 17.10% 21.00% 35.00%  
Statutory Federal tax rate         21.00% 35.00% 35.00%
State taxes, net of Federal tax benefit         0.60% 0.40% 0.30%
Depreciation of flow-through differences         (0.10%) 1.40% 2.10%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent         (0.30%) (0.40%) (0.30%)
Effective Income Tax Rate Reconciliation, Depreciation of flow-through years         (4.00%) 0.00% 0.00%
Other, net         (0.20%) (1.30%) (5.10%)
Effective tax rate   16.80%   16.20% 17.00% 35.10% 32.00%
Effective Income Tax Rate Reconciliation, at Combined Federal and State Statutory Income Tax Rate, Percent     21.60%