Accounting Policies (Claims and Insurance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Workers' Compensation [Member]
LIability For Unpaid Claims And Claim Adjustment Expense, Adjustment Of Opening Balance [Roll Forward]
Undiscounted amount	$ 299.4	$ 315.5
Estimated settlement cost	95.7	89.4
Claim payments, net of recoveries	(90.3)	(103.6)
Change in estimated settlement cost for older claim years	(5.5)	(1.9)
Undiscounted amount	299.3	299.4
Discounted settlement cost estimate	276.4
Property Damage and Liability Claims [Member]
LIability For Unpaid Claims And Claim Adjustment Expense, Adjustment Of Opening Balance [Roll Forward]
Undiscounted amount	72.9	84.9
Estimated settlement cost	37.2	30.0
Claim payments, net of recoveries	(33.5)	(51.5)
Change in estimated settlement cost for older claim years	(6.1)	9.5
Undiscounted amount	70.5	72.9
Discounted settlement cost estimate	69.3
Total [Member]
LIability For Unpaid Claims And Claim Adjustment Expense, Adjustment Of Opening Balance [Roll Forward]
Undiscounted amount	372.3	400.4
Estimated settlement cost	132.9	119.4
Claim payments, net of recoveries	(123.8)	(155.1)
Change in estimated settlement cost for older claim years	(11.6)	7.6
Undiscounted amount	369.8	372.3
Discounted settlement cost estimate	345.7
Cargo Claims And Other Insurance Related Amounts [Member]
LIability For Unpaid Claims And Claim Adjustment Expense, Adjustment Of Opening Balance [Roll Forward]
Undiscounted amount	15.3
Undiscounted amount	$ 15.0	$ 15.3