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Summary of Significant Accounting Policies (Related Party Transactions) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Charges for services provided $ 9.8us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty $ 10.4us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty  
Net Prepaid/Payables to the Service Company 3.0us-gaap_AccountsPayableRelatedPartiesCurrent (4.7)us-gaap_AccountsPayableRelatedPartiesCurrent  
Service Company [Member]      
Charges for services provided     35.8us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty
/ dei_LegalEntityAxis
= dpl_ServiceCompanyMember
Charges to the Service Company     0.1us-gaap_RelatedPartyTransactionOtherRevenuesFromTransactionsWithRelatedParty
/ dei_LegalEntityAxis
= dpl_ServiceCompanyMember
Net Prepaid/Payables to the Service Company     $ (4.7)us-gaap_AccountsPayableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= dpl_ServiceCompanyMember