Inventories (Details) - Entity [Domain] - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Inventories
Raw materials	$ 518,096	$ 518,096	$ 764,883
Supplies	385,728	385,728	374,599
Work in progress	110,750	110,750	128,882
Finished goods	277,495	277,495	350,055
Total inventories	1,292,069	1,292,069	$ 1,618,419
Minnesota Ironmaking Operations | Cost of goods sold
Inventories
Inventory lower-of-cost or market charge	21,000	21,000
Minnesota Ironmaking Operations | Noncontrolling Interests | Cost of goods sold
Inventories
Inventory lower-of-cost or market charge	$ 3,600	$ 3,600