Segment Information - Other Footnote (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Operating income (loss)
Corporate SG & A	$ (96,853)	$ (82,660)	$ (184,383)	$ (159,010)
Profit sharing	(20,176)	(5,031)	(29,467)	(9,629)
Operating income (loss)	256,122	77,559	388,087	177,330
Assets
Cash and equivalents	1,052,666	419,401	1,052,666	419,401	$ 976,951	$ 727,032	$ 160,982	$ 361,363
Accounts receivable	794,003		794,003			613,605
Inventories	1,175,716		1,175,716			1,149,390
Property, plant and equipment, net	2,885,844		2,885,844			2,951,210
Other	33,624		33,624			36,501
Total assets	6,629,676	6,826,763	6,629,676	6,826,763		$ 6,202,082
Operating Segment | Other
Operating income (loss)
Corporate SG & A	(14,800)	(8,100)	(25,900)	(17,500)
Company-wide equity- based compensation	(7,300)	(6,300)	(14,300)	(12,200)
Profit sharing	(18,500)	(3,500)	(26,700)	(7,000)
Other, net	(1,000)	(1,900)	(2,300)	(6,100)
Operating income (loss)	(45,569)	(52,970)	(77,499)	(88,872)
Assets
Cash and equivalents	954,800	356,900	954,800	356,900
Accounts receivable	21,000	29,000	21,000	29,000
Inventories	33,200	48,300	33,200	48,300
Property, plant and equipment, net	299,200	315,700	299,200	315,700
Intra-company debt	6,300	6,600	6,300	6,600
Other	33,800	113,400	33,800	113,400
Total assets	1,348,290	869,929	1,348,290	869,929
Operating Segment | Other | MINNESOTA | Minnesota ironmaking operations
Operating income (loss)
Operating income (loss)	$ (4,000)	$ (33,200)	$ (8,300)	$ (46,100)