Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Regulatory Assets [Line Items]
Regulatory assets	$ 294.1	$ 251.6
Regulatory Liabilities [Line Items]
Regulatory liabilities	287.6	247.5
Negative salvage
Regulatory Liabilities [Line Items]
Regulatory liabilities	241.7	203.8
Deferred cash out
Regulatory Liabilities [Line Items]
Regulatory liabilities	1.6	6.2
Sentinel meter station depreciation
Regulatory Liabilities [Line Items]
Regulatory liabilities	5.0	3.9
Postretirement benefits other than pension
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.3	24.7
Electric power cost
Regulatory Liabilities [Line Items]
Regulatory liabilities	13.8	7.7
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.2	1.2
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets	80.6	83.5
Asset retirement obligations
Regulatory Assets [Line Items]
Regulatory assets	128.5	125.1
Asset retirement costs - Eminence
Regulatory Assets [Line Items]
Regulatory assets	68.2	0
Deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	8.1	9.1
Postretirement benefits other than pension
Regulatory Assets [Line Items]
Regulatory assets	0	4.6
Fuel cost
Regulatory Assets [Line Items]
Regulatory assets	0.7	29.3
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 8.0	$ 0