BALANCE SHEETS (PARENTHETICAL) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Accounts receivable and unbilled revenues, reserves (in dollars)	$ 4.2	$ 4.2
Property, plant, and equipment, accumulated depreciation (in dollars)	$ 1,672.1	$ 1,620.5
Common stock, par value (in dollars per share)	$ 4
Common stock, shares authorized	32,000,000
Common stock, shares issued	23,896,962	23,896,962
Common stock, shares outstanding	23,896,962	23,896,962