Inventories (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Inventories
Raw materials	$ 559,996	$ 764,883
Supplies	385,347	374,599
Work in progress	114,718	128,882
Finished goods	261,336	350,055
Total inventories	1,321,397	$ 1,618,419
Minnesota Ironmaking Operations | Cost of goods sold
Inventories
Inventory lower-of-cost or market charge	21,000
Minnesota Ironmaking Operations | Noncontrolling Interests | Cost of goods sold
Inventories
Inventory lower-of-cost or market charge	$ 3,600