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Income Taxes (Effective and Statutory Rate Reconciliation) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Entity Information [Line Items]      
Non-cash capital contribution $ 40.0 $ 97.1 $ 0.0
Estimated Annual Effective Income Tax Rate, Continuing Operations 21.00% 35.00%  
Statutory Federal tax rate 21.00% 35.00% 35.00%
State taxes, net of Federal tax benefit 0.40% (1.50%) 0.20%
Depreciation of flow-through differences (0.10%) 4.90% (5.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation and Amortization, Percent (14.60%) (17.60%) 26.70%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent (1.00%) 5.10% (3.30%)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent 0.00% 11.00% (75.10%)
Effective Income Tax Rate Reconciliation, Permanent Difference, Percent 0.00% 4.80% 2.80%
Other, net (3.50%) 35.20% (0.70%)
Effective tax rate 2.20% 76.90% (19.40%)
THE DAYTON POWER AND LIGHT COMPANY [Member]      
Entity Information [Line Items]      
Estimated Annual Effective Income Tax Rate, Continuing Operations 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 17.00% 35.10% 32.00%