Income Taxes - Tax Effects of Significant Temporary Differences Creating Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits	$ 3.8	$ 4.2
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Postretirement Benefits	17.1	15.9
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation	4.4	4.3
Deferred Tax Assets, Inventory	7.6	6.2
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Pensions	3.2	7.4
Deferred Tax Assets, Tax Credit Carryforwards	10.3	10.7
Deferred Tax Assets, Operating Loss Carryforwards	8.8	2.0
Deferred tax assets, gross	89.7	56.2
Deferred Tax Assets, Valuation Allowance	5.3	3.8
Deferred Tax Assets, Net of Valuation Allowance	84.4	52.4
Deferred tax liabilities:
Plant and equipment	(179.2)	(161.8)
Deferred Tax Liabilities, Leasing Arrangements	(18.9)	0.0
Intangible assets	(3.8)	(5.6)
Deferred Tax Liabilities, Gross	201.9	167.4
Deferred Tax Liabilities, Net	117.5	115.0
Interest Expense [Member]
Deferred Other Tax Expense (Benefit)	9.9
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Tax Deferred Expense, Other	12.4	2.5
Lease Agreements [Member]
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Other	20.5	0.0
Other Expense [Member]
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Other	$ 1.6	$ 3.0