Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory assets	$ 316.9	$ 294.1
Regulatory Liabilities [Line Items]
Regulatory liabilities	333.1	287.6
Negative salvage
Regulatory Liabilities [Line Items]
Regulatory liabilities	283.8	241.7
Deferred cash out
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	1.6
Sentinel meter station depreciation
Regulatory Liabilities [Line Items]
Regulatory liabilities	5.8	5.0
Postretirement benefits other than pension
Regulatory Liabilities [Line Items]
Regulatory liabilities	39.1	25.3
Electric power cost
Regulatory Liabilities [Line Items]
Regulatory liabilities	4.4	13.8
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	0.2
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets	78.4	80.6
Asset retirement obligations
Regulatory Assets [Line Items]
Regulatory assets	115.9	128.5
Asset retirement costs - Eminence
Regulatory Assets [Line Items]
Regulatory assets	63.2	68.2
Deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	7.0	8.1
Deferred cash out
Regulatory Assets [Line Items]
Regulatory assets	12.9	0
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory assets	8.7	6.2
Fuel cost
Regulatory Assets [Line Items]
Regulatory assets	29.2	0.7
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 1.6	$ 1.8