Annual report pursuant to Section 13 and 15(d)

SEGMENT REPORTING SEGMENTS REPORTING (Revenue by Geographical Location) (Details)

v2.4.1.9
SEGMENT REPORTING SEGMENTS REPORTING (Revenue by Geographical Location) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Revenues $ 1,284.7us-gaap_Revenues $ 1,298.2us-gaap_Revenues $ 1,100.8us-gaap_Revenues $ 940.0us-gaap_Revenues $ 1,515.8us-gaap_Revenues $ 1,546.6us-gaap_Revenues $ 1,488.5us-gaap_Revenues $ 1,140.5us-gaap_Revenues $ 4,623.7us-gaap_Revenues $ 5,691.4us-gaap_Revenues $ 5,872.7us-gaap_Revenues
PROPERTY, PLANT AND EQUIPMENT, NET 1,414.9us-gaap_PropertyPlantAndEquipmentNet       11,153.4us-gaap_PropertyPlantAndEquipmentNet       1,414.9us-gaap_PropertyPlantAndEquipmentNet 11,153.4us-gaap_PropertyPlantAndEquipmentNet 11,207.3us-gaap_PropertyPlantAndEquipmentNet
United States [Member]                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Revenues                 2,139.4us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_US
1,857.6us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_US
2,108.5us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_US
PROPERTY, PLANT AND EQUIPMENT, NET 1,093.7us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_US
      2,721.6us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_US
      1,093.7us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_US
2,721.6us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_US
2,795.3us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_US
AUSTRALIA                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
PROPERTY, PLANT AND EQUIPMENT, NET 72.4us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_AU
      751.0us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_AU
      72.4us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_AU
751.0us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_AU
1,042.4us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_AU
China [Member]                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Revenues                 1,049.0us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_CN
1,909.4us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_CN
2,008.2us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_CN
Canada [Member]                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Revenues                 439.1us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_CA
871.2us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_CA
728.1us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_CA
PROPERTY, PLANT AND EQUIPMENT, NET 248.8us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_CA
      7,680.8us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_CA
      248.8us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_CA
7,680.8us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_CA
7,369.6us-gaap_PropertyPlantAndEquipmentNet
/ us-gaap_StatementGeographicalAxis
= country_CA
Other Countries [Member]                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Revenues                 $ 996.2us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= clf_OtherCountriesMember
$ 1,053.2us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= clf_OtherCountriesMember
$ 1,027.9us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= clf_OtherCountriesMember