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
	Mar. 31, 2013	Mar. 31, 2012
Change in unrealized gain / (loss)	$ (16.5)	$ (6.6)
Derivative, Gain (Loss) on Derivative, Net	0.9	(6.6)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(15.6)	(13.2)
NYMEX Coal Contract [Member]
Change in unrealized gain / (loss)		(7.8)
Derivative, Gain (Loss) on Derivative, Net		(5.0)
Derivative Instruments, Gain (Loss) Recognized in Income, Net		(12.8)
Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)		(0.1)
Derivative, Gain (Loss) on Derivative, Net	0.5	(0.2)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.5	(0.3)
Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)		(0.1)
Derivative, Gain (Loss) on Derivative, Net		0.9
Derivative Instruments, Gain (Loss) Recognized in Income, Net		0.8
Forward Contract Power [Member]
Change in unrealized gain / (loss)	(16.5)	1.4
Derivative, Gain (Loss) on Derivative, Net	0.4	(2.3)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(16.1)	(0.9)
DP&L [Member]
Change in unrealized gain / (loss)	(10.4)	(8.0)
Derivative, Gain (Loss) on Derivative, Net	1.2	(4.3)
Gain (Loss) on Derivative Instruments, Net, Pretax	(9.2)	(12.3)
Partner's share of gain (loss)		(3.5)
Regulatory (asset)/liability		(1.0)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(9.2)	(12.3)
DP&L [Member] | NYMEX Coal Contract [Member]
Change in unrealized gain / (loss)		(7.8)
Derivative, Gain (Loss) on Derivative, Net		(5.0)
Gain (Loss) on Derivative Instruments, Net, Pretax		(12.8)
Partner's share of gain (loss)		(3.5)
Regulatory (asset)/liability		(1.1)
Derivative Instruments, Gain (Loss) Recognized in Income, Net		(12.8)
DP&L [Member] | Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)		(0.1)
Derivative, Gain (Loss) on Derivative, Net	0.5	(0.2)
Gain (Loss) on Derivative Instruments, Net, Pretax	0.5	(0.3)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.5	(0.3)
DP&L [Member] | Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)		(0.1)
Derivative, Gain (Loss) on Derivative, Net		0.9
Gain (Loss) on Derivative Instruments, Net, Pretax		0.8
Regulatory (asset)/liability		0.1
Derivative Instruments, Gain (Loss) Recognized in Income, Net		0.8
DP&L [Member] | Forward Contract Power [Member]
Change in unrealized gain / (loss)	(10.4)
Derivative, Gain (Loss) on Derivative, Net	0.7
Gain (Loss) on Derivative Instruments, Net, Pretax	(9.7)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(9.7)
Revenue [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		3.4
Revenue [Member] | Forward Contract Power [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		3.4
Revenue [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(1.1)	1.6
Revenue [Member] | DP&L [Member] | Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(1.1)	1.6
Fuel [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		(7.6)
Fuel [Member] | NYMEX Coal Contract [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		(8.2)
Fuel [Member] | Commodity Contract - Heating Oil [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		0.6
Fuel [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net		(7.6)
Fuel [Member] | DP&L [Member] | NYMEX Coal Contract [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net		(8.2)
Fuel [Member] | DP&L [Member] | Commodity Contract - Heating Oil [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net		0.6
O&M [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		0.1
O&M [Member] | Commodity Contract - Heating Oil [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax		0.1
O&M [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net		0.1
O&M [Member] | DP&L [Member] | Commodity Contract - Heating Oil [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net		0.1
Purchased Power [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax	(15.6)	(4.6)
Purchased Power [Member] | Commodity Contract - FTR [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax	0.5	(0.3)
Purchased Power [Member] | Forward Contract Power [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax	(16.1)	(4.3)
Purchased Power [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(8.1)	(1.9)
Purchased Power [Member] | DP&L [Member] | Commodity Contract - FTR [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.5	(0.3)
Purchased Power [Member] | DP&L [Member] | Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(8.6)	(1.6)
Partners Share of Gain Loss [Member]
Partner's share of gain (loss)		(3.5)
Partners Share of Gain Loss [Member] | NYMEX Coal Contract [Member]
Partner's share of gain (loss)		(3.5)
Regulatory Asset Liability [Member]
Regulatory (asset)/liability		(1.0)
Regulatory Asset Liability [Member] | NYMEX Coal Contract [Member]
Regulatory (asset)/liability		(1.1)
Regulatory Asset Liability [Member] | Commodity Contract - Heating Oil [Member]
Regulatory (asset)/liability		$ 0.1