Fair Value Measurements (Fair Value of Assets and Liabilities Measured on a Nonrecurring Basis) (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Mar. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	$ 2,546,800,000			$ 2,546,800,000		$ 2,517,600,000
Fixed asset impairment	0	$ 0		0	$ 11,500,000
Goodwill	317,000,000			317,000,000		$ 317,000,000
Goodwill impairment	$ 0	0		0	135,800,000
DPLER [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill		135,800,000			135,800,000
Goodwill impairment			$ 135,800,000	0	135,800,000
East Bend Station [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value		14,200,000			14,200,000
Fair value			2,700,000
Fixed asset impairment			$ 11,500,000	$ 0	11,500,000
Level 3 [Member] | East Bend Station [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value		$ 2,700,000			$ 2,700,000