Condensed Consolidated Balance Sheets Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 6.0	$ 5.9
Common, stated value per share	$ 20	$ 20
Authorized	50,000,000	50,000,000
Issued to voting trustees	18,121,975	17,754,923
Issued to shareholders	3,815,159	3,744,318
In treasury, at cost	(334,322)	(58,172)
Outstanding common stock	21,602,812	21,441,069