Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets	$ 385.2	$ 373.5
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,032.0	1,007.1
Negative salvage
Regulatory Liabilities [Line Items]
Regulatory liabilities	444.5	409.7
Deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	450.2	471.1
Sentinel meter station depreciation
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.4	6.3
Postretirement benefits other than pension
Regulatory Liabilities [Line Items]
Regulatory liabilities	79.8	73.9
Electric power cost
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.1	13.3
Pension- deferred collections
Regulatory Liabilities [Line Items]
Regulatory liabilities	48.5	32.5
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	2.5	0.3
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets	39.0	37.9
Asset retirement obligations
Regulatory Assets [Line Items]
Regulatory assets	171.9	168.7
Asset retirement costs - Eminence
Regulatory Assets [Line Items]
Regulatory assets	45.5	49.5
Deferred taxes
Regulatory Assets [Line Items]
Regulatory assets	2.7	3.8
Deferred cash out
Regulatory Assets [Line Items]
Regulatory assets	54.9	42.5
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory assets	4.0	6.0
Fuel Cost
Regulatory Assets [Line Items]
Regulatory assets	61.2	61.4
Other
Regulatory Assets [Line Items]
Regulatory assets	$ 6.0	$ 3.7