Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Depreciation expense expressed as a percentage of average depreciable property	3.50%	3.40%	3.30%
Operating revenues
Minimum annual tolerance band percentage for future rate recovery or refund of fuel costs	2.00%
Cash and Cash Equivalents [Abstract]
Maximum number of months of remaining maturity at time of purchase to consider investments in certain instruments as cash equivalents	three months
Accounts, Notes, Loans and Financing Receivable, Classified [Abstract]
Allowance for Doubtful Accounts Receivable, Current	$ 5.6	$ 4.9
Alternative Revenue Programs [Abstract]
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs	24 months
Percentage of Average Annual Operating Revenues	1.20%
Number of Years Annual Operating Revenues are Averaged	3 years
Public Utilities, Inventory [Line Items]
Public Utilities, Inventory	$ 17.1	17.8
Materials and supplies
Public Utilities, Inventory [Line Items]
Public Utilities, Inventory	6.7	6.9
Fuel
Public Utilities, Inventory [Line Items]
Public Utilities, Inventory	3.8	3.9
Natural gas
Public Utilities, Inventory [Line Items]
Public Utilities, Inventory	$ 6.6	$ 7.0