Accounting Policies - Schedule of Undiscounted Amounts and Material Changes in Insurance Claims (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Liability For Claims And Claims Adjustment Expense [Line Items]
Undiscounted amount	$ 359.4	$ 361.4
Estimated settlement cost	131.4	138.5
Claim payments, net of recoveries	(141.7)	(132.0)
Change in estimated settlement cost for prior claim years	(3.9)	(8.5)
Undiscounted amount	345.2	359.4
Discounted settlement cost estimate	323.3
Workers’ Compensation
Liability For Claims And Claims Adjustment Expense [Line Items]
Undiscounted amount	285.6	287.8
Estimated settlement cost	95.0	103.3
Claim payments, net of recoveries	(88.1)	(95.9)
Change in estimated settlement cost for prior claim years	(17.3)	(9.6)
Undiscounted amount	275.2	285.6
Discounted settlement cost estimate	254.9
Third-Party Liability Claims
Liability For Claims And Claims Adjustment Expense [Line Items]
Undiscounted amount	73.8	73.6
Estimated settlement cost	36.4	35.2
Claim payments, net of recoveries	(53.6)	(36.1)
Change in estimated settlement cost for prior claim years	13.4	1.1
Undiscounted amount	70.0	$ 73.8
Discounted settlement cost estimate	$ 68.4