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Derivative Instruments and Hedging Activities - Part 4 (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Credit Risk-Related Contingent Features [Line Items]          
Gain on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring $ 0 $ 0 $ 0 $ 6,000,000  
DPL Subsidiary [Member]          
Credit Risk-Related Contingent Features [Line Items]          
Net liability position, derivative transactions 25,000,000   25,000,000   $ 12,000,000
Collateral posted for derivative transactions 6,000,000   6,000,000   5,000,000
Cash Collateral Received From Counterparties $ 0   $ 0   $ 0