Description of the Business and Significant Accounting Policies (Goodwill) (Details) - USD ($)	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Goodwill
Goodwill	$ 530,716,000	$ 429,645,000
OmniSource
Goodwill
Goodwill decrease due to tax benefit related to the normal amortization of the component	847,000
Steel Operations \| Columbus Flat Roll Division
Goodwill
Goodwill	19,682,000	19,682,000
Steel Operations \| The Techs
Goodwill
Goodwill	142,783,000	142,783,000
Steel Operations \| Heartland Flat Roll Division [Member]
Goodwill
Goodwill	46,143,000	46,143,000
Steel Operations \| United Steel Supply, LLC
Goodwill
Goodwill	101,918,000
Steel Operations \| Vulcan Steel Products [Member]
Goodwill
Goodwill	7,824,000	7,824,000
Steel Operations \| Roanoke Bar Division
Goodwill
Goodwill	29,041,000	29,041,000
Metals Recycling Operations \| OmniSource
Goodwill
Goodwill	181,400,000	182,247,000
Steel Fabrication Operations \| New Millennium Building Systems
Goodwill
Goodwill	$ 1,925,000	$ 1,925,000

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

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Amount of increase (decrease) of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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