Segment Information - Results Footnote (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating income (loss)
Corporate SG & A									$ (436,498)	$ (416,640)	$ (394,631)
Company-wide equity-based compensation									(43,300)	(38,700)	(36,600)
Profit sharing									(78,029)	(155,985)	(91,309)
Operating income (loss)	$ 181,961	$ 228,045	$ 285,032	$ 291,842	$ 365,553	$ 531,572	$ 501,887	$ 323,397	986,880	1,722,409	1,066,881
Gross profit	316,409	$ 359,839	$ 421,166	$ 433,570	521,235	$ 686,081	$ 652,082	$ 463,416	1,530,984	2,322,814	1,582,014
Assets
Cash and equivalents	1,381,460				828,220				1,381,460	828,220
Short-term Investments	262,174				228,783				262,174	228,783
Inventories	1,689,043				1,859,168				1,689,043	1,859,168
Property, plant and equipment, net	3,135,886				2,945,767				3,135,886	2,945,767
Other assets	106,038				25,166				106,038	25,166
Total assets	8,275,765				7,703,563				8,275,765	7,703,563
Operating Segment | Other
Operating income (loss)
Corporate SG & A									(66,100)	(58,000)	(49,400)
Company-wide equity-based compensation									(40,600)	(36,400)	(36,500)
Profit sharing									(73,600)	(149,800)	(87,600)
Other, net									(5,900)	(9,000)	(4,500)
Operating income (loss)									(186,159)	(253,195)	(190,785)
Assets
Cash and equivalents	1,298,800				811,100				1,298,800	811,100
Short-term Investments	262,200				198,800				262,200	198,800
Accounts receivable	17,100				5,900				17,100	5,900
Inventories	37,000				33,700				37,000	33,700
Property, plant and equipment, net	149,800				153,800				149,800	153,800
Intra-company debt	8,100				14,000				8,100	14,000
Other assets	75,700				66,200				75,700	66,200
Total assets	1,848,711				1,283,539				1,848,711	1,283,539
Operating Segment | Other | MINNESOTA | Minnesota ironmaking operations
Operating income (loss)
Operating income (loss)											(12,800)
Operating Segment | Metals Recycling Operations
Operating income (loss)
Operating income (loss)									16,308	75,891	71,052
Assets
Total assets	940,250				946,689				940,250	946,689
Eliminations
Operating income (loss)
Operating income (loss)									7,078	(2,040)	689
Gross profit									6,200		$ 700
Assets
Accounts receivable	(54,700)				(64,600)				(54,700)	(64,600)
Intra-company debt	(8,100)				(14,000)				(8,100)	(14,000)
Other assets	(7,900)				(14,100)				(7,900)	(14,100)
Total assets	$ (70,707)				$ (92,671)				$ (70,707)	$ (92,671)