Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory assets	$ 343.3	$ 316.9
Regulatory Liabilities [Line Items]
Regulatory liabilities	385.9	333.1
Negative salvage
Regulatory Liabilities [Line Items]
Regulatory liabilities	318.3	283.8
Sentinel meter station depreciation
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.0	5.8
Postretirement benefits other than pension
Regulatory Liabilities [Line Items]
Regulatory liabilities	51.0	39.1
Electric power cost
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.8	4.4
Pension- deferred collections
Regulatory Liabilities [Line Items]
Regulatory liabilities	8.0	0.0
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	1.8	0.0
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets	75.8	78.4
Asset retirement obligations
Regulatory Assets [Line Items]
Regulatory assets	113.5	115.9
Asset retirement costs - Eminence
Regulatory Assets [Line Items]
Regulatory assets	58.8	63.2
Deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	5.9	7.0
Deferred cash out
Regulatory Assets [Line Items]
Regulatory assets	43.9	12.9
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory assets	0.0	8.7
Fuel cost
Regulatory Assets [Line Items]
Regulatory assets	43.8	29.2
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 1.6	$ 1.6