Segment Information - Other Footnote (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Operating income (loss)
Corporate SG & A	$ (95,185)	$ (82,648)	$ (279,899)	$ (242,207)
Profit sharing	(22,255)	(9,008)	(51,722)	(18,637)
Operating income (loss)	283,855	131,029	671,942	308,359
Assets
Cash and equivalents	1,051,489	473,790	1,051,489	473,790	$ 1,052,666	$ 727,032	$ 419,401	$ 361,363
Accounts receivable	779,459		779,459			613,605
Inventories	1,275,575		1,275,575			1,149,390
Property, plant and equipment, net	2,928,226		2,928,226			2,951,210
Other	31,011		31,011			36,501
Total assets	6,787,562	6,842,541	6,787,562	6,842,541		$ 6,202,082
Other
Operating income (loss)
Corporate SG & A	(12,600)	(9,500)	(38,500)	(27,100)
Company-wide equity-based compensation	(6,100)	(5,300)	(20,400)	(17,500)
Profit sharing	(21,000)	(7,500)	(47,700)	(14,400)
Other, net	(3,900)	(2,000)	(6,200)	(8,000)
Operating income (loss)	(47,687)	(28,401)	(125,186)	(117,273)
Assets
Cash and equivalents	982,200	415,700	982,200	415,700
Accounts receivable	14,700	30,000	14,700	30,000
Inventories	36,200	36,800	36,200	36,800
Property, plant and equipment, net	295,700	309,600	295,700	309,600
Intra-company debt	12,100	6,500	12,100	6,500
Other	33,600	(100,200)	33,600	(100,200)
Total assets	1,374,469	898,772	1,374,469	898,772
Other | MINNESOTA | Minnesota ironmaking operations
Operating income (loss)
Operating income (loss)	$ (4,100)	$ (4,100)	$ (12,400)	$ (50,300)