Derivative Instruments and Hedging Activities (Classification within the Condensed Consolidated Statements of Results of Operations or Balance Sheets of the Gains and Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Change in unrealized gain / (loss)	$ 17.2	$ 5.1	$ 9.2	$ (1.4)
Derivative, Gain (Loss) on Derivative, Net	1.5	(10.4)	2.5	(17.0)
Gain (Loss) on Derivative Instruments, Net, Pretax			11.7	(18.4)
Partner's share of gain (loss)				(1.2)
Regulatory (asset)/liability				(0.9)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	18.7	(5.3)	11.7	(18.4)
NYMEX Coal Contract [Member]
Change in unrealized gain / (loss)		5.7		(2.0)
Derivative, Gain (Loss) on Derivative, Net		(9.5)		(14.5)
Gain (Loss) on Derivative Instruments, Net, Pretax				(16.5)
Partner's share of gain (loss)				(1.2)
Regulatory (asset)/liability				(0.3)
Derivative Instruments, Gain (Loss) Recognized in Income, Net		(3.8)		(16.5)
Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	(0.9)	(0.2)	(0.9)	(0.2)
Derivative, Gain (Loss) on Derivative, Net	0.6	0.7	1.2	0.5
Gain (Loss) on Derivative Instruments, Net, Pretax			0.3	0.3
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.3)	0.5	0.3	0.3
Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)	(0.3)	(1.3)	(0.3)	(1.5)
Derivative, Gain (Loss) on Derivative, Net		0.5		1.4
Gain (Loss) on Derivative Instruments, Net, Pretax			(0.3)	(0.1)
Regulatory (asset)/liability				(0.6)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.3)	(0.8)	(0.3)	(0.1)
Forward Contract Power [Member]
Change in unrealized gain / (loss)	18.4	0.9	10.4	2.3
Derivative, Gain (Loss) on Derivative, Net	0.9	(2.1)	1.3	(4.4)
Gain (Loss) on Derivative Instruments, Net, Pretax			11.7	(2.1)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	19.3	(1.2)	11.7	(2.1)
DP&L [Member]
Change in unrealized gain / (loss)	16.6	5.1	7.9	(2.8)
Derivative, Gain (Loss) on Derivative, Net	1.8	(8.3)	3.1	(12.5)
Gain (Loss) on Derivative Instruments, Net, Pretax	18.4	(3.2)	11.0	(15.3)
Partner's share of gain (loss)		2.3		(1.2)
Regulatory (asset)/liability	(0.1)	0.2		(0.9)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	18.4	(3.2)	11.0	(15.3)
DP&L [Member] | NYMEX Coal Contract [Member]
Change in unrealized gain / (loss)		5.7		(2.0)
Derivative, Gain (Loss) on Derivative, Net		(9.5)		(14.5)
Gain (Loss) on Derivative Instruments, Net, Pretax		(3.8)		(16.5)
Partner's share of gain (loss)		2.3		(1.2)
Regulatory (asset)/liability		0.8		(0.3)
Derivative Instruments, Gain (Loss) Recognized in Income, Net		(3.8)		(16.5)
DP&L [Member] | Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	(0.9)	(0.2)	(0.9)	(0.2)
Derivative, Gain (Loss) on Derivative, Net	0.6	0.7	1.2	0.5
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.3)	0.5	0.3	0.3
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.3)	0.5	0.3	0.3
DP&L [Member] | Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)	(0.3)	(1.3)	(0.3)	(1.5)
Derivative, Gain (Loss) on Derivative, Net		0.5		1.4
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.3)	(0.8)	(0.3)	(0.1)
Regulatory (asset)/liability	(0.1)	(0.6)		(0.6)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.3)	(0.8)	(0.3)	(0.1)
DP&L [Member] | Forward Contract Power [Member]
Change in unrealized gain / (loss)	17.8	0.9	9.1	0.9
Derivative, Gain (Loss) on Derivative, Net	1.2		1.9	0.1
Gain (Loss) on Derivative Instruments, Net, Pretax	19.0	0.9	11.0	1.0
Derivative Instruments, Gain (Loss) Recognized in Income, Net	19.0	0.9	11.0	1.0
Revenue [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		(2.7)
Derivative Instruments, Gain (Loss) Recognized in Income, Net				0.7
Revenue [Member] | Forward Contract Power [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		(2.7)
Derivative Instruments, Gain (Loss) Recognized in Income, Net				0.7
Revenue [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.1)	1.4	0.3	(0.5)
Revenue [Member] | DP&L [Member] | Commodity Contract - FTR [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net		0.5		0.3
Revenue [Member] | DP&L [Member] | Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.1)	0.9	0.3	(0.8)
Fuel [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.2)	(7.2)
Derivative Instruments, Gain (Loss) Recognized in Income, Net			(0.2)	(14.7)
Fuel [Member] | NYMEX Coal Contract [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		(6.9)
Derivative Instruments, Gain (Loss) Recognized in Income, Net				(15.0)
Fuel [Member] | Commodity Contract - Heating Oil [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.2)	(0.3)
Derivative Instruments, Gain (Loss) Recognized in Income, Net			(0.2)	0.3
Fuel [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.2)	(7.2)	(0.2)	(14.7)
Fuel [Member] | DP&L [Member] | NYMEX Coal Contract [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net		(6.9)		(15.0)
Fuel [Member] | DP&L [Member] | Commodity Contract - Heating Oil [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.2)	(0.3)	(0.2)	0.3
O&M [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		0.1
Derivative Instruments, Gain (Loss) Recognized in Income, Net				0.2
O&M [Member] | Commodity Contract - Heating Oil [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		0.1
Derivative Instruments, Gain (Loss) Recognized in Income, Net				0.2
O&M [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net		0.1		0.2
O&M [Member] | DP&L [Member] | Commodity Contract - Heating Oil [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net		0.1		0.2
Purchased Power [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax	19.0	2.0
Derivative Instruments, Gain (Loss) Recognized in Income, Net			12.0	(2.5)
Purchased Power [Member] | Commodity Contract - FTR [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.3)	0.5
Derivative Instruments, Gain (Loss) Recognized in Income, Net			0.3	0.3
Purchased Power [Member] | Forward Contract Power [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax	19.3	1.5
Derivative Instruments, Gain (Loss) Recognized in Income, Net			11.7	(2.8)
Purchased Power [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	18.8		11.0	1.8
Purchased Power [Member] | DP&L [Member] | Commodity Contract - FTR [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.3)		0.3
Purchased Power [Member] | DP&L [Member] | Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	19.1		10.7	1.8
Partners Share of Gain Loss [Member]
Partner's share of gain (loss)		2.3
Partners Share of Gain Loss [Member] | NYMEX Coal Contract [Member]
Partner's share of gain (loss)		2.3
Regulatory Asset Liability [Member]
Regulatory (asset)/liability	(0.1)	0.2	(0.1)
Regulatory Asset Liability [Member] | NYMEX Coal Contract [Member]
Regulatory (asset)/liability		0.8
Regulatory Asset Liability [Member] | Commodity Contract - Heating Oil [Member]
Regulatory (asset)/liability	(0.1)	(0.6)	(0.1)
Regulatory Asset Liability [Member] | DP&L [Member]
Regulatory (asset)/liability			(0.1)
Regulatory Asset Liability [Member] | DP&L [Member] | Commodity Contract - Heating Oil [Member]
Regulatory (asset)/liability			$ (0.1)