Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory assets	$ 344.7	$ 360.4
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,003.9	1,024.3
Negative salvage
Regulatory Liabilities [Line Items]
Regulatory liabilities	504.5	482.6
Deferred taxes - liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	392.8	423.5
Sentinel meter station depreciation
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.8	6.6
Postretirement benefits other than pension
Regulatory Liabilities [Line Items]
Regulatory liabilities	60.9	68.0
Pension- unamortized liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	38.9	43.3
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.0	0.3
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets	36.1	37.5
Asset retirement obligations
Regulatory Assets [Line Items]
Regulatory assets	139.5	147.6
Asset retirement costs - Eminence
Regulatory Assets [Line Items]
Regulatory assets	36.6	42.2
Deferred cash out
Regulatory Assets [Line Items]
Regulatory assets	70.0	64.4
Fuel Cost
Regulatory Assets [Line Items]
Regulatory assets	34.1	40.2
Electric power cost
Regulatory Assets [Line Items]
Regulatory assets	7.8	8.4
Slug catcher
Regulatory Assets [Line Items]
Regulatory assets	6.4	6.6
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 14.2	$ 13.5