Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory assets	$ 373.5	$ 351.1
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,007.1	458.5
Negative salvage
Regulatory Liabilities [Line Items]
Regulatory liabilities	409.7	357.1
Deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	471.1	0.0
Sentinel meter station depreciation
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.3	6.2
Postretirement benefits other than pension
Regulatory Liabilities [Line Items]
Regulatory liabilities	73.9	63.0
Electric power cost
Regulatory Liabilities [Line Items]
Regulatory liabilities	13.3	6.3
Pension- deferred collections
Regulatory Liabilities [Line Items]
Regulatory liabilities	32.5	21.3
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.3	4.6
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets	37.9	73.2
Asset retirement obligations
Regulatory Assets [Line Items]
Regulatory assets	168.7	117.2
Asset retirement costs - Eminence
Regulatory Assets [Line Items]
Regulatory assets	49.5	53.9
Deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	3.8	4.8
Deferred cash out
Regulatory Assets [Line Items]
Regulatory assets	42.5	48.2
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory assets	6.0	0.0
Fuel Cost
Regulatory Assets [Line Items]
Regulatory assets	61.4	51.3
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 3.7	$ 2.5