Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Accrued postretirement benefits cost	$ 15.4	$ 0.1
Accrued pension cost	57.4	55.1
Other employee benefit accruals	11.0	10.9
Tax loss and credit carryforwards	90.2	86.0
Other, net	46.7	46.9
Valuation allowances	(36.7)	(33.7)
Total deferred tax assets	184.0	165.3
Deferred tax liabilities - principally depreciation and amortization	(255.7)	(261.6)
Net deferred tax (liabilities) assets	$ (71.7)	$ (96.3)