Summary of Significant Accounting Policies, Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Significant accounting policies
Inventory Write-down	$ 0.0	$ 0.0	$ 4.9	$ 0.0	$ 4.9
Workers Compensation Liability Undiscounted	26.6			$ 26.6		$ 26.8
Workers' Compensation Discount, Percent				2.00%		2.00%
Accrued liabilities for employee healthcare benefits	3.3			$ 3.3		$ 3.6
Fabricated Products
Significant accounting policies
Excess of current cost over the stated LIFO value of inventory	$ (12.5)			$ (12.5)		$ 8.5