Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Long Term Debt Range Adjusted To Fair Value at Merger Start	2013
Long Term Debt Range Adjusted To Fair Value At Merger End	2061
Unrealized gains / (losses) on financial instruments, Amount before tax	$ 0.7	$ 0.7
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax		0.5
Unrealized gains and immaterial unrealized losses in AOCI, after tax	(0.5)
Unrealized gains or losses are expected to be transferred to earnings in the next twelve months.	0.1
Unrealized gains or losses expected to be realized over next twelve months, net of tax	(0.1)
Percent of inputs to the fair value of derivative instruments from quoted market prices	96.00%
Money market funds	35.0	130.0
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds	2.1
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax	(1.4)
DP&L [Member]
Long Term Debt Range Adjusted To Fair Value at Merger Start	2013
Long Term Debt Range Adjusted To Fair Value At Merger End	2061
Unrealized gains / (losses) on financial instruments, Amount before tax		1.6
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax		1.0
Unrealized gains and immaterial unrealized losses in AOCI, before tax	1.0
Unrealized gains and immaterial unrealized losses in AOCI, after tax	(0.7)
Unrealized gains or losses are expected to be transferred to earnings in the next twelve months.	0.1
Unrealized gains or losses expected to be realized over next twelve months, net of tax	(0.1)
Percent of inputs to the fair value of derivative instruments from quoted market prices	96.00%
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds	2.1
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax	$ (1.4)