Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation Allowance [Line Items]
Total gross unrecognized tax benefits	$ 38.9	$ 26.0	$ 14.0	$ 39.2
Current Fiscal Year End Date	--12-31
Deferred tax assets:
Accrued postretirement benefits cost	$ 0.1	28.9
Accrued pension cost	55.1	59.5
Other employee benefit accruals	10.9	16.8
Deferred Tax Assets Tax Loss and Credit Carryforwards	86.0	86.1
Other, net	46.9	42.9
Valuation allowances	(33.7)	(77.5)
Total deferred tax assets	165.3	156.7
Deferred Tax Liabilities, Gross	261.6	235.7
Deferred tax liabilities - principally depreciation and amortization	$ 96.3	$ 79.0