Segment Information - Other Footnote (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating income (loss)
Corporate SG & A									$ (374,009)	$ (328,758)	$ (317,275)
Profit sharing									(71,285)	(23,064)	(42,126)
Asset impairment charges									132,839	428,500	260,000
Operating income (loss)	$ 56,024	$ 283,855	$ 256,122	$ 131,965	$ (381,143)	$ 131,029	$ 77,559	$ 99,771	727,966	(72,784)	320,320
Assets
Cash and equivalents	841,483				727,032				841,483	727,032	361,363	$ 395,156
Accounts receivable	729,784				613,605				729,784	613,605
Inventories	1,275,211				1,149,390				1,275,211	1,149,390
Property, plant and equipment, net	2,787,215				2,951,210				2,787,215	2,951,210
Other	29,514				36,501				29,514	36,501
Total assets	6,423,732				6,202,082				6,423,732	6,202,082
Minnesota Ironmaking Operations
Operating income (loss)
Asset impairment charges									127,300	428,500	260,000
Other
Operating income (loss)
Corporate SG & A									(48,900)	(37,800)	(43,200)
Company-wide equity-based compensation									(30,400)	(27,100)	(22,800)
Profit sharing									(66,400)	(17,700)	(36,800)
Other, net									(10,500)	(10,500)	(12,900)
Operating income (loss)									(301,116)	(148,784)	(445,549)
Assets
Cash and equivalents	775,700				641,900				775,700	641,900
Accounts receivable	2,000				18,800				2,000	18,800
Inventories	35,400				35,100				35,400	35,100
Income tax receivable	25,300								25,300
Property, plant and equipment, net	168,500				306,000				168,500	306,000
Intra-company debt	12,100				6,200				12,100	6,200
Other	35,400				42,500				35,400	42,500
Total assets	1,054,448				1,050,506				1,054,448	1,050,506
Business Combination, Acquisition Related Costs											25,200
Other | MINNESOTA | Minnesota Ironmaking Operations
Operating income (loss)
Asset impairment charges									(129,100)		(260,000)
Operating income (loss)									(15,800)	(55,700)	(69,800)
Metals Recycling Operations
Operating income (loss)
Operating income (loss)									20,423	(448,137)	$ 26,014
Assets
Total assets	$ 1,038,946				$ 1,067,672				1,038,946	$ 1,067,672
Goodwill, Impairment Loss									$ 5,500