Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) employee	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) mi² employee customer segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment			1
Cash and Cash Equivalents, at Carrying Value	$ 43.5		$ 43.5		$ 36.5
Entity number of employees | employee	631		631
Employees under a collective bargaining agreement which expires in October-2011			59.00%
Excise taxes collected	$ 13.4	$ 13.0	$ 36.7	$ 37.3
Restricted Cash and Cash Equivalents, Current	0.1		0.1		10.5
Restricted Cash and Cash Equivalents	43.6	54.4	$ 43.6	54.4	47.0	$ 111.7
Subsidiaries [Member]
Significant Accounting Policies [Line Items]
Number of reportable segments | segment			1
Cash and Cash Equivalents, at Carrying Value	$ 26.1		$ 26.1		10.8
Approximate number of retail customers | customer			530,000
Service area, square miles | mi²			6,000
Number of Operating Segments | segment			1
Entity number of employees | employee	630		630
Employees under a collective bargaining agreement which expires in October-2011			59.00%
Excise taxes collected	$ 13.4	13.0	$ 36.7	37.3
Restricted Cash and Cash Equivalents, Current	0.1		0.1		10.5
Restricted Cash and Cash Equivalents	$ 26.2	$ 32.0	$ 26.2	$ 32.0	$ 21.3	$ 66.2