Segments - Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net sales	$ 6,460,774	$ 5,174,769	$ 12,029,193	$ 9,989,948
Earnings (loss) before income taxes and noncontrolling interests	913,701	508,136	1,429,613	1,057,111
Total assets	17,451,692		17,451,692		$ 15,841,258
Operating Segments [Member] | Steel Mills [Member]
Segment Reporting Information [Line Items]
Net sales	4,169,539	3,404,064	7,750,233	6,594,571
Earnings (loss) before income taxes and noncontrolling interests	961,784	579,520	1,522,287	1,223,703
Total assets	8,590,433		8,590,433		7,671,217
Operating Segments [Member] | Steel Products [Member]
Segment Reporting Information [Line Items]
Net sales	1,738,370	1,366,693	3,207,081	2,579,050
Earnings (loss) before income taxes and noncontrolling interests	155,766	83,636	241,580	150,555
Total assets	4,672,730		4,672,730		4,323,907
Operating Segments [Member] | Raw Materials [Member]
Segment Reporting Information [Line Items]
Net sales	552,865	404,012	1,071,879	816,327
Earnings (loss) before income taxes and noncontrolling interests	134,995	66,227	209,542	92,618
Total assets	3,573,219		3,573,219		3,396,110
Corporate and Eliminations Items [Member]
Segment Reporting Information [Line Items]
Net sales	(4,271,955)	(3,245,261)	(7,814,995)	(6,111,961)
Earnings (loss) before income taxes and noncontrolling interests	(338,844)	(221,247)	(543,796)	(409,765)
Total assets	615,310		615,310		$ 450,024
Intercompany Eliminations [Member] | Steel Mills [Member]
Segment Reporting Information [Line Items]
Net sales	1,065,780	759,245	1,964,106	1,417,605
Intercompany Eliminations [Member] | Steel Products [Member]
Segment Reporting Information [Line Items]
Net sales	50,907	26,664	86,677	56,365
Intercompany Eliminations [Member] | Raw Materials [Member]
Segment Reporting Information [Line Items]
Net sales	$ 3,155,268	$ 2,459,352	$ 5,764,212	$ 4,637,991