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Description of the Business and Summary of Significant Accounting Policies (Goodwill) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 01, 2019
Goodwill
Goodwill	$ 452,915	$ 429,645
Goodwill impairment loss	0	0	$ 0
Compensation expense for stock-based employee compensation plan	43,300	38,700	$ 36,600
United Steel Supply, LLC
Goodwill
Business Acquisition, Percentage of Voting Interests Acquired				75.00%
Steel Operations
Goodwill
Goodwill	272,133	245,473
Steel Operations \| United Steel Supply, LLC
Goodwill
Goodwill	26,700
Metals Recycling Operations
Goodwill
Goodwill	178,857	182,247
Metals Recycling Operations \| OmniSource
Goodwill
Goodwill decrease due to tax benefit related to the normal amortization of the component	3,400
Goodwill impaired accumulated impairment loss	346,800	346,800
Steel Fabrication Operations
Goodwill
Goodwill	$ 1,925	$ 1,925