Revenue - Schedule of Net Sales Disaggregates by Major Source (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Revenue from Contract with Customers [Line Items]
Revenues	$ 6,460,774	$ 12,029,193
Sheet [Member]
Revenue from Contract with Customers [Line Items]
Revenues	1,974,427	3,640,647
Bar [Member]
Revenue from Contract with Customers [Line Items]
Revenues	1,258,438	2,348,585
Structural [Member]
Revenue from Contract with Customers [Line Items]
Revenues	448,557	845,254
Plate [Member]
Revenue from Contract with Customers [Line Items]
Revenues	488,117	915,747
Tubular Products [Member]
Revenue from Contract with Customers [Line Items]
Revenues	371,568	682,796
Rebar Fabrication [Member]
Revenue from Contract with Customers [Line Items]
Revenues	390,921	720,140
Other Steel Products [Member]
Revenue from Contract with Customers [Line Items]
Revenues	975,881	1,804,145
Raw Materials [Member]
Revenue from Contract with Customers [Line Items]
Revenues	552,865	1,071,879
Steel Mills [Member]
Revenue from Contract with Customers [Line Items]
Revenues	4,169,539	7,750,233
Steel Mills [Member] | Sheet [Member]
Revenue from Contract with Customers [Line Items]
Revenues	1,974,427	3,640,647
Steel Mills [Member] | Bar [Member]
Revenue from Contract with Customers [Line Items]
Revenues	1,258,438	2,348,585
Steel Mills [Member] | Structural [Member]
Revenue from Contract with Customers [Line Items]
Revenues	448,557	845,254
Steel Mills [Member] | Plate [Member]
Revenue from Contract with Customers [Line Items]
Revenues	488,117	915,747
Steel Products [Member]
Revenue from Contract with Customers [Line Items]
Revenues	1,738,370	3,207,081
Steel Products [Member] | Tubular Products [Member]
Revenue from Contract with Customers [Line Items]
Revenues	371,568	682,796
Steel Products [Member] | Rebar Fabrication [Member]
Revenue from Contract with Customers [Line Items]
Revenues	390,921	720,140
Steel Products [Member] | Other Steel Products [Member]
Revenue from Contract with Customers [Line Items]
Revenues	975,881	1,804,145
Raw Materials [Member]
Revenue from Contract with Customers [Line Items]
Revenues	552,865	1,071,879
Raw Materials [Member] | Raw Materials [Member]
Revenue from Contract with Customers [Line Items]
Revenues	$ 552,865	$ 1,071,879