Customer Concentrations - Sales to Significant Customers (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Concentration Risk [Line Items]
Sales and other operating revenue	$ 343.6	$ 336.2	$ 347.6	$ 323.9	$ 384.8	$ 376.2	$ 371.7	$ 358.0	$ 1,351.3	$ 1,490.7	$ 1,633.5
ArcelorMittal | Sales Revenue, Net | Customer Concentration Risk
Concentration Risk [Line Items]
Sales and other operating revenue									$ 662.3	$ 771.9	$ 826.7
Sales to customers (as a percent)									49.00%	51.80%	50.60%
AK Steel | Sales Revenue, Net | Customer Concentration Risk
Concentration Risk [Line Items]
Sales and other operating revenue									$ 395.4	$ 402.4	$ 489.7
Sales to customers (as a percent)									29.30%	27.00%	30.00%
U.S. Steel | Sales Revenue, Net | Customer Concentration Risk
Concentration Risk [Line Items]
Sales and other operating revenue									$ 212.7	$ 249.2	$ 276.6
Sales to customers (as a percent)									15.70%	16.70%	16.90%