Regulatory Assets and Liabilities (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 915.9	$ 899.1
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	728.2	723.9
Escrowed Bad Debt Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	69.0	18.5
Other, net [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 118.7	$ 156.7