Accounting Policies - Schedule of Undiscounted Amounts and Material Changes in Insurance Claims (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Liability For Claims And Claims Adjustment Expense [Line Items]
Undiscounted amount	$ 361.4	$ 369.8
Estimated settlement cost	138.5	135.9
Claim payments, net of recoveries	(132.0)	(128.6)
Change in estimated settlement cost for prior claim years	(8.5)	(15.7)
Undiscounted amount	359.4	361.4
Discounted settlement cost estimate	335.6
Workers’ Compensation
Liability For Claims And Claims Adjustment Expense [Line Items]
Undiscounted amount	287.8	299.3
Estimated settlement cost	103.3	95.9
Claim payments, net of recoveries	(95.9)	(92.4)
Change in estimated settlement cost for prior claim years	(9.6)	(15.0)
Undiscounted amount	285.6	287.8
Discounted settlement cost estimate	262.2
Third-Party Liability Claims
Liability For Claims And Claims Adjustment Expense [Line Items]
Undiscounted amount	73.6	70.5
Estimated settlement cost	35.2	40.0
Claim payments, net of recoveries	(36.1)	(36.2)
Change in estimated settlement cost for prior claim years	1.1	(0.7)
Undiscounted amount	73.8	$ 73.6
Discounted settlement cost estimate	$ 73.4