XML 91 R65.htm IDEA: XBRL DOCUMENT
v3.8.0.1

INVENTORIES (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Inventory [Line Items] |  |
Cost of Goods Sold | $ 6.2 | $ 8.8
LCM inventory adjustment | 1.8 | 0.0
U.S. Iron Ore [Member] |  |
Inventory [Line Items] |  |
Inventory, LIFO Reserve | $ 96.2 | $ 78.5

X
- Definition Effect of inventory reduction + References No definition available. + Details
Name: | clf_Effectofinventoryreduction
Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition LCM inventory adjustment + References No definition available. + Details
Name: | clf_LCMinventoryadjustment
Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount by which inventory stated at last-in first-out (LIFO) is less than (in excess of) inventory stated at other inventory cost methods. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(c)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_InventoryLIFOReserve
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_InventoryLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=clf_USIronOreMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |