Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets
Pension liabilities		$ 56.2	$ 74.5
Retirement benefits other than pensions		71.3	87.2
Workers’ compensation and other claims		29.1	41.7
Property and equipment, net		5.2	6.6
Other assets and liabilities		88.6	107.3
Net operating loss carryforwards		41.1	42.4
Alternative minimum and other tax credits(a)		68.2	62.0
Subtotal		359.7	421.7
Valuation allowances		(98.9)	(62.8)	$ (45.7)	$ (40.1)
Total deferred tax assets		260.8	358.9
Deferred tax liabilities
Property and equipment, net		3.7	0.0
Retirement benefits other than pensions		2.0	2.1
Other assets and miscellaneous		54.0	36.5
Deferred tax liabilities		59.7	38.6
Net deferred tax asset		201.1	320.3
Included in:
Noncurrent assets		226.2	327.9
Noncurrent liabilities		(25.1)	(7.6)
Net deferred tax asset		201.1	$ 320.3
Operating Loss Carryforwards [Line Items]
Foreign tax credit amount	$ 64.2
Remaining credits		4.0
Foreign
Deferred tax assets
Net operating loss carryforwards		$ 24.6
Operating Loss Carryforwards [Line Items]
Carryforward period (in years)		10 years
Tax Cuts and Jobs Act of 2017
Operating Loss Carryforwards [Line Items]
Foreign tax credit amount	$ 31.1