Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Customer receivables, allowance for doubtful accounts	$ 1.7	$ 1.7
Common stock , par value (in dollars per share)	$ 2.50	$ 2.50
Common stock, authorized (in shares)	50,000,000	50,000,000
Common stock, outstanding (in shares)	9,200,000	9,200,000