Equity Investments - Additional Information (Detail)	3 Months Ended			1 Months Ended		3 Months Ended		6 Months Ended				3 Months Ended		3 Months Ended
	Apr. 05, 2014 USD ($)	Mar. 30, 2013 USD ($)	Dec. 31, 2013 USD ($)	Apr. 05, 2014 Hunter Ridge Energy Services LLC [Member] USD ($)	Dec. 31, 2013 Hunter Ridge Energy Services LLC [Member] USD ($)	Apr. 05, 2014 Duferdofin Nucor S.R.L. [Member] USD ($)	Mar. 30, 2013 Duferdofin Nucor S.R.L. [Member] USD ($)	Jun. 30, 2012 Duferdofin Nucor S.R.L. [Member] USD ($)	Apr. 05, 2014 Duferdofin Nucor S.R.L. [Member] EUR (€)	Dec. 31, 2013 Duferdofin Nucor S.R.L. [Member] USD ($)	Dec. 31, 2013 Duferdofin Nucor S.R.L. [Member] EUR (€)	Apr. 05, 2014 NuMit [Member] USD ($)	Dec. 31, 2013 NuMit [Member] USD ($)	Apr. 05, 2014 Steel Technologies LLC [Member] USD ($) Sheet
Schedule of Equity Method Investments [Line Items]
Equity method investment	$ 944,900,000		$ 936,000,000	$ 135,400,000	$ 134,500,000	$ 463,600,000				$ 465,400,000		$ 326,900,000	$ 318,400,000
Period used for lag basis, in months				1 month		1 month						1 month
Equity method investment, ownership percentage				50.00%		50.00%						50.00%		100.00%
Equity method investments, share of net assets						55,000,000
Step-up to fair value of certain assets and liabilities						408,600,000
Step-up to fair value of equity method investment, portion related to identification of goodwill						331,000,000
Amortization expense and other purchase accounting adjustments due to fair value step-up	18,432,000	19,048,000				2,700,000	2,900,000
Due from related parties, noncurrent						48,000,000			35,000,000	48,200,000	35,000,000
Notes receivable, related parties, interest rate						1.539%								1.13%
Interest rate per year in excess of Euribor as of date of the notes						1.00%
Equity method investment's note payable with parent company, maturity date						Jan. 31, 2016								Oct. 21, 2014
Equity method investment's credit facilities, amount									122,500,000
Total amount outstanding under equity method investment's credit facilities						163,800,000			119,500,000	154,400,000	112,000,000
Line of credit facility, maturity period						Apr. 26, 2016								Apr. 01, 2015
Guarantor obligation percentage of exposure in case of default						50.00%
Number of sheet processing facilities operated by Steel Technologies														24
Notes receivable, related parties														40,000,000
Basis points in excess of LIBOR on note receivable from related party														0.90%
Line of credit extended to joint venture, amount outstanding														14,000,000
Line of credit extended to joint venture														100,000,000
Amended line of credit extended to joint venture														60,000,000
Line of credit extended to joint venture, interest rate														1.36%
Impairment of equity method investments								$ 30,000,000