Summary of Significant Accounting Policies, Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Significant accounting policies
Inventory Write-down	$ 0.0	$ 0.0	$ 4.9	$ 4.9	$ 0.0
WorkersCompensationLiabilityUndiscounted	25.5			$ 25.5		$ 23.5
Workers' Compensation Discount, Percent				1.75%		1.75%
Accrued liabilities for employee healthcare benefits	3.3			$ 3.3		$ 3.2
Adjustments for ASU 2015-03
Significant accounting policies
Reclassification of deferred financing costs upon adoption of ASU 2015-03	3.2			3.2
Fabricated Products
Significant accounting policies
(Shortfalls) excess of current cost over the stated LIFO value of inventory	$ 11.2			$ 11.2		$ 24.1