Summary of Significant Accounting Policies - Property, Plants and Equipment (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Capitalized interest costs	$ 3.7	$ 3.2	$ 1.0
Service Life | Indiana Harbor
Property, Plant and Equipment [Line Items]
Depreciation	$ 10.2	$ 15.6	$ 9.5
Depreciation per common share (in USD per share)	$ 0.16	$ 0.23	$ 0.14
Coke and energy plant, machinery and equipment | Minimum
Property, Plant and Equipment [Line Items]
Coke and energy plant, machinery and equipment useful life (in years)	25 years
Coke and energy plant, machinery and equipment | Maximum
Property, Plant and Equipment [Line Items]
Coke and energy plant, machinery and equipment useful life (in years)	30 years
Coal logistics plant, machinery and equipment | Minimum
Property, Plant and Equipment [Line Items]
Coke and energy plant, machinery and equipment useful life (in years)	15 years
Coal logistics plant, machinery and equipment | Maximum
Property, Plant and Equipment [Line Items]
Coke and energy plant, machinery and equipment useful life (in years)	35 years