Financing Arrangements - Long-term debt (Details) - USD ($) $ in Millions		12 Months Ended
	Jun. 18, 2010	Dec. 31, 2018	Dec. 31, 2017
Financing Arrangements
Total debt		$ 355.0	$ 499.5
Less debt issuance costs (deduction from liability)		(1.6)	(2.4)
Less current maturities		(30.0)	(22.5)
Total long-term debt		323.4	474.6
Principal payments during each of the next five years and thereafter
2019		30.0
2020		105.0
2021		220.0
2022		0.0
Thereafter		0.0
Letters of credit outstanding		$ 25.8	25.7
5.05% Senior notes due 2020
Financing Arrangements
Interest rate (as a percent)		5.05%
Total debt		$ 75.0	75.0
Amount drawn	$ 75.0
5.05% Senior notes due 2020 | Minimum
Financing Arrangements
Optional amount that the Company may prepay		1.0
Term Loan due February 2021
Financing Arrangements
Total debt		255.0	277.5
Line of Credit matures February 2021
Financing Arrangements
Total debt		$ 25.0	$ 147.0
Letters of credit
Financing Arrangements
Term of letters of credit from the date of issuance		1 year