Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets	$ 351.1	$ 343.3
Regulatory Liabilities [Line Items]
Regulatory liabilities	458.5	385.9
Negative salvage
Regulatory Liabilities [Line Items]
Regulatory liabilities	357.1	318.3
Sentinel meter station depreciation
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.2	6.0
Postretirement benefits other than pension
Regulatory Liabilities [Line Items]
Regulatory liabilities	63.0	51.0
Electric power cost
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.3	0.8
Pension- deferred collections
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.3	8.0
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	4.5	1.6
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.1	0.2
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets	73.2	75.8
Asset retirement obligations
Regulatory Assets [Line Items]
Regulatory assets	117.2	113.5
Asset retirement costs - Eminence
Regulatory Assets [Line Items]
Regulatory assets	53.9	58.8
Deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	4.8	5.9
Deferred cash out
Regulatory Assets [Line Items]
Regulatory assets	48.2	43.9
Fuel Cost
Regulatory Assets [Line Items]
Regulatory assets	51.3	43.8
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 2.5	$ 1.6