The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

+ Details

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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_RelatedPartyTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=clf_HibbingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=clf_ArcelorMittalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=clf_USSteelCanadaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	clf_NameofOperationAxis=clf_EmpireMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	clf_NameofOperationAxis=clf_TildenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

RELATED PARTIES (Summary Of Other Ownership Interests) (Details)	Jun. 30, 2017	Dec. 31, 2016
Hibbing [Member] \| Arcelor Mittal [Member]
Related Party Transaction [Line Items]
Ownership interest, equity method investment	62.30%
Hibbing [Member] \| U. S. Steel Canada [Member]
Related Party Transaction [Line Items]
Ownership interest, equity method investment	14.70%
Empire [Member]
Related Party Transaction [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent	79.00%
Empire [Member] \| Arcelor Mittal [Member]
Related Party Transaction [Line Items]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	21.00%
Empire [Member] \| U. S. Steel Canada [Member]
Related Party Transaction [Line Items]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	0.00%
Tilden [Member]
Related Party Transaction [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent	85.00%
Tilden [Member] \| Arcelor Mittal [Member]
Related Party Transaction [Line Items]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	0.00%
Tilden [Member] \| U. S. Steel Canada [Member]
Related Party Transaction [Line Items]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	15.00%
Other Noncurrent Assets [Member] \| Hibbing [Member]
Related Party Transaction [Line Items]
Ownership interest, equity method investment	23.00%	23.00%