Segment Information - Results Footnote (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating income (loss)
Corporate SG&A									$ (416,640)	$ (394,631)	$ (374,009)
Profit sharing									(155,985)	(91,309)	(71,285)
Asset impairment charge											132,839
Operating income (loss)	$ 365,553	$ 531,572	$ 501,887	$ 323,397	$ 195,947	$ 271,015	$ 265,353	$ 334,566	1,722,409	1,066,881	727,966
Gross Profit	521,235	$ 686,081	$ 652,082	$ 463,416	320,824	$ 396,518	$ 392,518	$ 472,154	2,322,814	1,582,014	1,334,864
Assets
Cash and equivalents	828,220				1,028,649				828,220	1,028,649
Short-term Investments	228,783								228,783
Accounts receivable	1,043,756				868,837				1,043,756	868,837
Inventories	1,859,168				1,519,347				1,859,168	1,519,347
Property, plant and equipment, net	2,945,767				2,675,904				2,945,767	2,675,904
Other assets	25,166				27,684				25,166	27,684
Total assets	7,703,563				6,855,732				7,703,563	6,855,732
Minnesota ironmaking operations
Operating income (loss)
Asset impairment charge											127,300
Operating Segment | MINNESOTA | Minnesota ironmaking operations
Operating income (loss)
Asset impairment charge											127,300
Eliminations
Operating income (loss)
Asset impairment charge											1,800
Operating income (loss)									(2,040)	689	(9,250)
Gross Profit									(2,000)	700	(11,100)
Assets
Accounts receivable	(64,600)				(106,100)				(64,600)	(106,100)
Intra-company debt	(14,000)				(13,400)				(14,000)	(13,400)
Other assets	(14,100)				(12,000)				(14,100)	(12,000)
Total assets	(92,671)				(131,496)				(92,671)	(131,496)
Steel Operations | Operating Segment
Operating income (loss)
Operating income (loss)									1,839,852	1,098,630	926,954
Assets
Total assets	5,150,298				4,257,167				5,150,298	4,257,167
Metals Recycling Operations | Operating Segment
Operating income (loss)
Asset impairment charge											5,500
Operating income (loss)									75,891	71,052	20,423
Assets
Total assets	946,689				1,011,298				946,689	1,011,298
Steel Fabrication Operations | Operating Segment
Operating income (loss)
Operating income (loss)									61,901	87,295	90,955
Assets
Total assets	415,708				384,402				415,708	384,402
Other | Operating Segment
Operating income (loss)
Corporate SG&A									(58,000)	(49,400)	(48,900)
Company-wide equity-based compensation									(36,400)	(36,500)	(30,400)
Profit sharing									(149,800)	(87,600)	(66,400)
Other, net									(9,000)	(4,500)	(10,500)
Operating income (loss)									(253,195)	(190,785)	(301,116)
Assets
Cash and equivalents	811,100				1,003,200				811,100	1,003,200
Short-term Investments	198,800								198,800
Accounts receivable	5,900				9,400				5,900	9,400
Inventories	33,700				58,800				33,700	58,800
Income tax receivable					57,300					57,300
Property, plant and equipment, net	153,800				160,900				153,800	160,900
Intra-company debt	14,000				13,400				14,000	13,400
Other assets	66,200				31,400				66,200	31,400
Total assets	$ 1,283,539				$ 1,334,361				$ 1,283,539	1,334,361
Other | Operating Segment | MINNESOTA | Minnesota ironmaking operations
Operating income (loss)
Asset impairment charge											(129,100)
Operating income (loss)										$ (12,800)	$ (15,800)