Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory assets	$ 360.4	$ 385.2
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,024.3	1,032.0
Negative salvage
Regulatory Liabilities [Line Items]
Regulatory liabilities	482.6	444.5
Deferred taxes - liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	423.5	450.2
Sentinel meter station depreciation
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.6	6.4
Postretirement benefits other than pension
Regulatory Liabilities [Line Items]
Regulatory liabilities	68.0	79.8
Pension- unamortized liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	43.3	48.5
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.3	2.6
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets	37.5	39.0
Asset retirement obligations
Regulatory Assets [Line Items]
Regulatory assets	147.6	171.9
Asset retirement costs - Eminence
Regulatory Assets [Line Items]
Regulatory assets	42.2	45.5
Deferred cash out
Regulatory Assets [Line Items]
Regulatory assets	64.4	54.9
Fuel Cost
Regulatory Assets [Line Items]
Regulatory assets	40.2	61.2
Electric power cost
Regulatory Assets [Line Items]
Regulatory assets	8.4	0.0
Slug catcher
Regulatory Assets [Line Items]
Regulatory assets	6.6	0.0
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 13.5	$ 12.7