Goodwill and Other Intangible Assets - Schedule of Change in Net Carrying Amount of Goodwill by Segment (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Goodwill [Line Items]
Balance, beginning of period	$ 2,196,058
Translation	(493)
Balance, end of period	2,195,565
Steel Mills [Member]
Goodwill [Line Items]
Balance, beginning of period	745,484
Reclassifications	(153,498)
Balance, end of period	591,986
Steel Products [Member]
Goodwill [Line Items]
Balance, beginning of period	720,997
Translation	(493)
Reclassifications	153,498
Balance, end of period	874,002
Raw Materials [Member]
Goodwill [Line Items]
Balance, beginning of period	729,577
Balance, end of period	$ 729,577