Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2013	Jun. 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
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax			0.5
Unrealized gains and immaterial unrealized losses in AOCI, before tax		0.2
Unrealized gains and immaterial unrealized losses in AOCI, after tax		(0.1)
Percent of inputs to the fair value of derivative instruments from quoted market prices		21.00%
Money market funds	0	0	130.0
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds	1.5
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax	(1.0)
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		0.7	1.6
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax		0.5	1.0
Percent of inputs to the fair value of derivative instruments from quoted market prices		84.00%
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds	1.5
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax	$ (1.0)

Realized gains and losses from sale of available for sale securities, net of tax.

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- Definition

Long Term Debt Range Adjusted To Fair Value At Merger End

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Long Term Debt Range Adjusted To Fair Value at Merger Start

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- Definition

Percentage Of Inputs To The Fair Value Of Derivative Instruments From Quoted Market Prices

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- Definition

Unrealized Gains and Immaterial Unrealized Losses in AOCI, Before Tax

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Unrealized Gains and Immaterial Unrealized Losses in AOCI, Net of Tax

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- Definition

The change in unrealized holding gain (loss), net of tax, on available-for-sale securities included in a separate component of shareholders' equity during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 21

-Subparagraph d

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-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

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- Definition

The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

+ References

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-Name Accounting Standards Codification

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-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

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- Definition

Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

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-Name Accounting Standards Codification

-Glossary Cash Equivalents

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- Definition

Before tax and reclassification adjustments amount of unrealized holding gain (loss) and transfers on available-for-sale securities.

+ References

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-Topic 220

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-Section 45

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-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph 38

-Subparagraph c(3)

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