Business Combination (Change to Preliminary Purchase Price Allocation Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	$ 7.5
Change Before Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	(94.5)
Total Adjustment [Member] | Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	(87.0)
Other Deferred Liabilities [Member] | Change Before Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	1.0
Taxes Payable [Member] | Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	(16.0)
Taxes Payable [Member] | Change Before Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	13.1
Regulatory Assets [Member] | Change Before Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	15.4
Deferred Tax Liabilities [Member] | Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	(20.7)
Out Of Market Coal Contract [Member] | Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	12.0
Out Of Market Coal Contract [Member] | Change Before Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	(34.2)
DPLER Intangibles [Member] | Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	6.7
DPLER Intangibles [Member] | Change Before Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	(19.1)
Property, Plant And Equipment [Member] | Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	25.5
Property, Plant And Equipment [Member] | Change Before Deferred Income Tax Effect [Member]
Goodwill, Purchase Accounting Adjustments	$ (70.7)