XML 106 R80.htm IDEA: XBRL DOCUMENT
v3.8.0.1

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Narrative) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |
401(k) contribution match | $ 0.50 |
Minimum participants percentage | 15.00% |
Defined Benefit Plan, Accumulated Benefit Obligation | $ 963,000,000 | $ 922,000,000
Reserve for investment commitments | $ 52,700,000 |
Structured Finance [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Redemption request notice period, days | 90 days |
Real Estate [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Redemption request notice period, days | 45 days |
Withdrawal request notice period, days | 65 days |
Pension Benefits [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Decrease in Projected Benefit Obligation due to Market Conditions | $ 46,100,000 |
Reduction in net periodic benefit expense | 8,200,000 |
Increase in Projected Benefit Obligation due to new mortality tables | $ 6,100,000 | $ 13,100,000
Percentage Increase in Projected Benefit Obligation due to new mortality tables | 0.60% | 1.40%
Other Benefits [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant | $ 4,600,000 | $ 6,000,000
Decrease in Projected Benefit Obligation due to Market Conditions | 12,600,000 |
Reduction in net periodic benefit expense | 1,800,000 |
Increase in Projected Benefit Obligation due to new mortality tables | $ 1,900,000 | $ 4,900,000
Percentage Increase in Projected Benefit Obligation due to new mortality tables | 0.70% | 1.80%
Prior To Age65 [Member] | United States [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Annual limit on medical coverage for each participant | $ 7,000 |
Prior To Age65 [Member] | Minimum [Member] | Northshore [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Annual limit on medical coverage for each participant | 4,020 |
Prior To Age65 [Member] | Maximum [Member] | Northshore [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Annual limit on medical coverage for each participant | $ 4,500 |

X
- Definition Decrease in Projected Benefit Obligation due to Market Conditions + References No definition available. + Details
Name: | clf_DecreaseinProjectedBenefitObligationduetoMarketConditions
Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Deferred Compensation Arrangement with Individual, Written Notice Period Required for Withdrawals + References No definition available. + Details
Name: | clf_DeferredCompensationArrangementWithIndividualWrittenNoticePeriodRequiredForWithdrawals
Namespace Prefix: | clf_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Increase in Projected Benefit Obligation due to new mortality tables + References No definition available. + Details
Name: | clf_IncreaseinProjectedBenefitObligationduetonewmortalitytables
Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Minimum Participants' Cost Percentage to Average Coverage Cost + References No definition available. + Details
Name: | clf_MinimumParticipantsCostPercentageToAverageCoverageCost
Namespace Prefix: | clf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage Increase in Projected Benefit Obligation due to new mortality tables + References No definition available. + Details
Name: | clf_PercentageIncreaseinProjectedBenefitObligationduetonewmortalitytables
Namespace Prefix: | clf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Postretirement Benefits Medical Coverage, Annual Limit + References No definition available. + Details
Name: | clf_PostretirementBenefitsMedicalCoverageAnnualLimit
Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Redemption Request Notice Period + References No definition available. + Details
Name: | clf_RedemptionRequestNoticePeriod
Namespace Prefix: | clf_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reduction in net periodic benefit expense + References No definition available. + Details
Name: | clf_Reductioninnetperiodicbenefitexpense
Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Reserve for Investment Commitments + References No definition available. + Details
Name: | clf_ReserveForInvestmentCommitments
Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation
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_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of contributions received by defined benefit plan from participant which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanPlanAssetsContributionsByPlanParticipant
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_StructuredFinanceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_RealEstateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | clf_CoverageAgeAxis=clf_PriorToAge65Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=clf_NorthshoreMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |