Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Total Master Trust Assets, Fair Value	$ 10.6		$ 10.3
DP&L [Member]
Total Master Trust Assets, Fair Value	10.6		10.3
Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	10.6	[1]	10.3	[1]
Total Derivative Assets	7.5	[1]	6.0	[1]
Total Assets	18.1	[1]	16.3	[1]
Total Derivative Liabilities	(11.7)	[1]	(19.5)	[1]
Total Liabilities	(938.6)	[1]	(19.5)	[1]
Level 1 [Member]
Total Master Trust Assets, Fair Value	0.2
Total Derivative Assets	0.2		1.8
Total Assets			1.8
Total Assets	0.4
Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.2
Total Derivative Assets	0.2		1.8
Total Assets	0.4		1.8
Level 2 [Member]
Total Master Trust Assets, Fair Value	10.4		10.3
Total Derivative Assets	6.3		17.4
Total Assets			27.7
Total Assets	16.7
Total Derivative Liabilities	(42.6)		(60.3)
Total Liabilities	(2,730.8)		(60.3)
Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	10.4		10.3
Total Derivative Assets	7.3		4.2
Total Assets	17.7		14.5
Total Derivative Liabilities	(11.6)		(19.5)
Total Liabilities	(919.6)		(19.5)
Level 3 [Member]
Total Derivative Liabilities	(0.1)
Total Liabilities	(19.0)
Level 3 [Member] | DP&L [Member]
Total Derivative Liabilities	(0.1)
Total Liabilities	(19.0)
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	10.6	[1]	10.3	[1]
Total Derivative Assets	6.5	[1]	19.2	[1]
Total Assets			29.5	[1]
Total Assets	17.1	[1]
Total Derivative Liabilities	(42.7)	[1]	(60.3)	[1]
Total Liabilities	(2,749.8)	[1]	(60.3)	[1]
FTRs [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Derivative Assets			0.1	[1]
Total Derivative Liabilities	(0.1)	[1]
FTRs [Member] | Level 2 [Member]
Total Derivative Assets			0.1
FTRs [Member] | Level 2 [Member] | DP&L [Member]
Total Derivative Assets			0.1
FTRs [Member] | Level 3 [Member]
Total Derivative Liabilities	(0.1)
FTRs [Member] | Level 3 [Member] | DP&L [Member]
Total Derivative Liabilities	(0.1)
FTRs [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets			0.1	[1]
Total Derivative Liabilities	(0.1)	[1]
Futures [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Derivative Assets	0.2	[1]	1.8	[1]
Futures [Member] | Level 1 [Member]
Total Derivative Assets	0.2		1.8
Futures [Member] | Level 1 [Member] | DP&L [Member]
Total Derivative Assets	0.2		1.8
Futures [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets	0.2	[1]	1.8	[1]
Forward NYMEX Coal Contracts [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Derivative Liabilities			(14.5)	[1]
Forward NYMEX Coal Contracts [Member] | Level 2 [Member]
Total Derivative Liabilities			(14.5)
Forward NYMEX Coal Contracts [Member] | Level 2 [Member] | DP&L [Member]
Total Derivative Liabilities			(14.5)
Forward NYMEX Coal Contracts [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Liabilities			(14.5)	[1]
Interest Rate Hedge [Member] | Level 2 [Member]
Total Derivative Liabilities	(29.5)		(32.5)
Interest Rate Hedge [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Liabilities	(29.5)	[1]	(32.5)	[1]
Debt [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Debt Instrument, Fair Value Disclosure	(926.9)	[1]
Debt [Member] | Level 2 [Member]
Debt Instrument, Fair Value Disclosure	(2,688.2)
Debt [Member] | Level 2 [Member] | DP&L [Member]
Debt Instrument, Fair Value Disclosure	(908.0)
Debt [Member] | Level 3 [Member]
Debt Instrument, Fair Value Disclosure	(18.9)
Debt [Member] | Level 3 [Member] | DP&L [Member]
Debt Instrument, Fair Value Disclosure	(18.9)
Debt [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Debt Instrument, Fair Value Disclosure	(2,707.1)	[1]
Forward Contract Power [Member]
Total Derivative Assets	6.3
Fair Value Frozen	8.2
Forward Contract Power [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Derivative Assets	7.3	[1]	4.1	[1]
Total Derivative Liabilities	(11.6)	[1]	(5.0)	[1]
Forward Contract Power [Member] | Level 2 [Member]
Total Derivative Assets	6.3		17.3
Total Derivative Liabilities	(13.1)		(13.3)
Forward Contract Power [Member] | Level 2 [Member] | DP&L [Member]
Total Derivative Assets	7.3		4.1
Total Derivative Liabilities	(11.6)		(5.0)
Forward Contract Power [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets	6.3	[1]	17.3	[1]
Total Derivative Liabilities	(13.1)	[1]	(13.3)	[1]
Equity Securities [Member]
Total Master Trust Assets, Fair Value	5.1		4.4
Equity Securities [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	5.1		4.4
Equity Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	5.1	[1]	4.4	[1]
Equity Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	5.1		4.4
Equity Securities [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	5.1		4.4
Equity Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	5.1	[1]	4.4	[1]
Debt Securities [Member]
Total Master Trust Assets, Fair Value	5.0		5.5
Debt Securities [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	5.0		5.5
Debt Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	5.0	[1]	5.5	[1]
Debt Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	5.0		5.5
Debt Securities [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	5.0		5.5
Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	5.0	[1]	5.5	[1]
Money Market Funds [Member]
Total Master Trust Assets, Fair Value	0.2		0.2
Money Market Funds [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.2		0.2
Money Market Funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.2	[1]	0.2	[1]
Money Market Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.2
Money Market Funds [Member] | Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.2
Money Market Funds [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value			0.2
Money Market Funds [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value			0.2
Money Market Funds [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2	[1]	0.2	[1]
Hedge Funds, Multi-strategy [Member]
Total Master Trust Assets, Fair Value	0.3		0.2
Hedge Funds, Multi-strategy [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.3	[1]	0.2	[1]
Hedge Funds, Multi-strategy [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value			0.2
Hedge Funds, Multi-strategy [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.3		0.2
Hedge Funds, Multi-strategy [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value			0.2	[1]
Multi Strategy Fund [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.3		0.2
Multi Strategy Fund [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.3
Multi Strategy Fund [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	$ 0.3	[1]

[1] Includes credit valuation adjustment