Regulatory Assets and Liabilities (Details 1) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 830.6	$ 879.1
Deferred cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	741.1	724.5
Escrowed Bad Debt Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	30.1	64.6
Other, net [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 59.4	$ 90.0