Description of the Business and Summary of Significant Accounting Policies (Intangible Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 526,777	$ 578,653
Less accumulated amortization	269,868	294,676
Intangible Assets, Net (Excluding Goodwill), Total	256,909	283,977
Amortization of Intangible Assets	29,193	28,765	$ 24,180
Estimated amortization expense
2018	26,910
2019	24,823
2020	22,629
2021	20,840
2022	19,231
Thereafter	142,476
Total	$ 256,909
Impairment of Long-Lived Tangible and Definite-Lived Intangible Assets
Assets held for sale, asset impairment charge		10,300
Asset impairment charges		132,839	428,500
Weighted average
Finite-Lived Intangible Assets [Line Items]
Useful Life	20 years
Minnesota ironmaking operations
Impairment of Long-Lived Tangible and Definite-Lived Intangible Assets
Asset impairment charges	$ 127,300	5,500	$ 428,500
Minnesota ironmaking operations | Noncontrolling Interests
Impairment of Long-Lived Tangible and Definite-Lived Intangible Assets
Asset impairment charges	13,100
Minnesota ironmaking operations | Parent [Member]
Impairment of Long-Lived Tangible and Definite-Lived Intangible Assets
Asset impairment charges	72,900
Customer and scrap generator relationships
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	$ 394,062	446,688
Customer and scrap generator relationships | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful Life	5 years
Customer and scrap generator relationships | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful Life	25 years
Customer and scrap generator relationships | Weighted average
Finite-Lived Intangible Assets [Line Items]
Useful Life	21 years
Trade names
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	$ 130,550	130,550
Trade names | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful Life	15 years
Trade names | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful Life	25 years
Trade names | Weighted average
Finite-Lived Intangible Assets [Line Items]
Useful Life	18 years
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	$ 2,165	1,415
Other Intangible Assets [Member] | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful Life	3 years
Other Intangible Assets [Member] | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful Life	5 years
Other Intangible Assets [Member] | Weighted average
Finite-Lived Intangible Assets [Line Items]
Useful Life	4 years
Other current assets
Impairment of Long-Lived Tangible and Definite-Lived Intangible Assets
Assets held for sale	$ 8,700	$ 29,300