Segment Information - Other Footnote (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Operating income (loss)
Corporate SG & A	$ (87,530)	$ (76,350)
Profit sharing	(9,291)	(4,598)
Operating income (loss)	131,965	99,771
Assets
Cash and equivalents	976,951	160,982	$ 727,032	$ 361,363
Accounts receivable	689,201		613,605
Inventories	1,066,823		1,149,390
Property, plant and equipment, net	2,914,186		2,951,210
Other	34,823		36,501
Total assets	6,391,642		$ 6,202,082
Operating Segment | Other
Operating income (loss)
Corporate SG & A	(11,100)	(9,400)
Company-wide equity- based compensation	(7,000)	(5,900)
Profit sharing	(8,200)	(3,500)
Other, net	(1,300)	(4,100)
Operating income (loss)	(31,930)	(35,902)
Assets
Cash and equivalents	895,400
Accounts receivable	22,700
Inventories	30,200
Property, plant and equipment, net	302,600
Intra-company debt	6,800
Other	38,200
Total assets	1,295,859
Operating Segment | Other | MINNESOTA | Minnesota ironmaking operations
Operating income (loss)
Operating income (loss)	$ (4,300)	$ (13,000)