Goodwill and Other Intangible Assets - Schedule of Change in Net Carrying Amount of Goodwill by Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Balance, beginning of period	$ 2,201,063	$ 2,184,336
Acquisitions	19,663	12,623
Translation	8,946	4,104
Balance, end of period	2,229,672	2,201,063
Steel Mills [Member]
Goodwill [Line Items]
Balance, beginning of period	591,986	591,986
Acquisitions	20,484
Balance, end of period	612,470	591,986
Steel Products [Member]
Goodwill [Line Items]
Balance, beginning of period	879,500	862,773
Acquisitions	(821)	12,623
Translation	8,946	4,104
Balance, end of period	887,625	879,500
Raw Materials [Member]
Goodwill [Line Items]
Balance, beginning of period	729,577	729,577
Balance, end of period	$ 729,577	$ 729,577