CONDENSED BALANCE SHEETS (PARENTHETICAL) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Accounts receivable and unbilled revenues, reserves	$ 13.3	$ 18.3
Property, plant, and equipment, accumulated depreciation and amortization	$ 1,800.9	$ 1,750.7
Common stock, par value (in dollars per share)	$ 4	$ 4
Common stock, shares authorized	32,000,000	32,000,000
Common stock, shares issued	23,896,962	23,896,962
Common stock, shares outstanding	23,896,962	23,896,962