CONDENSED BALANCE SHEETS (PARENTHETICAL) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Statement of Financial Position [Abstract]
Accounts receivable and unbilled revenues, reserves	$ 8.3	$ 10.9
Property, plant, and equipment, accumulated depreciation and amortization	$ 2,112.9	$ 2,033.4
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