Regulatory Assets and Liabilities (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 868.3	$ 915.9
Regulatory Assets and Liabilities (Textuals)
Regulatory assets and liabilities, current expected to be amortization period	within one year
Deferred cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	725.0	728.2
Escrowed Bad Debt Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	81.1	69.0
Other, net [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 62.2	$ 118.7