Annual report pursuant to Section 13 and 15(d)

INVENTORIES (Narrative) (Details)

v2.4.1.9
INVENTORIES (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
U.S. Iron Ore [Member]      
Inventory [Line Items]      
Inventory, LIFO Reserve $ 119.0us-gaap_InventoryLIFOReserve
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember
$ 115.3us-gaap_InventoryLIFOReserve
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember
 
Effect of LIFO Inventory Liquidation on Income 44.8us-gaap_EffectOfLIFOInventoryLiquidationOnIncome
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember
   
Increase in Inventories due to build up of LIFO layers   7.4clf_IncreaseInInventoriesDueToBuildUpOfLifoLayers
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember
 
Cost of Sales [Member] | Eastern Canadian Iron Ore [Member]      
Inventory [Line Items]      
Inventory Write-down 38.9us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember
  0us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember
Cost of Sales [Member] | North American Coal [Member]      
Inventory [Line Items]      
Inventory Write-down 44.5us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_NorthAmericanCoalMember
11.1us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_NorthAmericanCoalMember
24.4us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_NorthAmericanCoalMember
Work in process inventory [Member] | Bloom Lake [Member] | Cost of Sales [Member]      
Inventory [Line Items]      
Inventory Write-down 0us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_PublicUtilitiesInventoryAxis
= clf_WorkinprocessinventoryMember
/ clf_ReportingUnitAxis
= clf_BloomLakeMember
   
Work in process inventory [Member] | Wabush [Member] | Cost of Sales [Member]      
Inventory [Line Items]      
Inventory Write-down 0us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_PublicUtilitiesInventoryAxis
= clf_WorkinprocessinventoryMember
/ clf_ReportingUnitAxis
= clf_WabushMember
   
Finished goods inventory [Member] | Bloom Lake [Member] | Cost of Sales [Member]      
Inventory [Line Items]      
Inventory Write-down 0us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_PublicUtilitiesInventoryAxis
= clf_FinishedgoodsinventoryMember
/ clf_ReportingUnitAxis
= clf_BloomLakeMember
   
Finished goods inventory [Member] | Wabush [Member] | Cost of Sales [Member]      
Inventory [Line Items]      
Inventory Write-down 0us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_PublicUtilitiesInventoryAxis
= clf_FinishedgoodsinventoryMember
/ clf_ReportingUnitAxis
= clf_WabushMember
   
Pellets [Member] | Cost of Sales [Member] | Eastern Canadian Iron Ore [Member]      
Inventory [Line Items]      
Inventory Write-down   11.1us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_PublicUtilitiesInventoryAxis
= clf_PelletsMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember
 
Pellets [Member] | Wabush [Member] | Cost of Sales [Member]      
Inventory [Line Items]      
Unsaleable Inventory Impairment Charge   10.6clf_UnsaleableInventoryImpairmentCharge
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_PublicUtilitiesInventoryAxis
= clf_PelletsMember
/ clf_ReportingUnitAxis
= clf_WabushMember
 
Concentrate [Member] | Cost of Sales [Member] | Eastern Canadian Iron Ore [Member]      
Inventory [Line Items]      
Inventory Write-down   13.2us-gaap_InventoryWriteDown
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_PublicUtilitiesInventoryAxis
= clf_ConcentrateMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember
 
Concentrate [Member] | Wabush [Member] | Cost of Sales [Member]      
Inventory [Line Items]      
Unsaleable Inventory Impairment Charge   $ 7.9clf_UnsaleableInventoryImpairmentCharge
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_PublicUtilitiesInventoryAxis
= clf_ConcentrateMember
/ clf_ReportingUnitAxis
= clf_WabushMember