Quarterly report pursuant to Section 13 or 15(d)

REVENUE (Narrative) (Details)

v3.8.0.1
REVENUE (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Jan. 01, 2018
Dec. 31, 2017
Revenues $ 239.0 $ 461.6    
Deferred Revenue, Current 31.0     $ 23.8
Deferred Revenue, Noncurrent 51.4     51.4
Other current liabilities 104.3   $ 87.6 86.2
Customer Supplemental Payments [Member]        
Deferred Revenue 64.2     64.2
Deferred Revenue, Current 12.8     12.8
Deferred Revenue, Noncurrent       51.4
Take or Pay Contracts [Member]        
Deferred Revenue 18.2     $ 9.6
U.S. Iron Ore [Member]        
Revenues 180.0 286.2    
U.S. Iron Ore [Member] | Derivative Adjustment [Member]        
Revenues 43.8      
Asia Pacific Iron Ore [Member]        
Revenues 59.0 $ 175.4    
Asia Pacific Iron Ore [Member] | Derivative Adjustment [Member]        
Revenues 1.3      
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]        
Revenues 62.5      
Other current liabilities $ 0.3   $ 1.4