Cleveland-Cliffs Inc. Annual report pursuant to Section 13 and 15(d)

SEGMENT REPORTING SEGMENTS REPORTING (Revenue by Geographical Location) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 754.0	$ 553.3	$ 496.2	$ 305.5	$ 476.0	$ 593.2	$ 498.1	$ 446.0	$ 2,109.0	$ 2,013.3	$ 3,373.2
PROPERTY, PLANT AND EQUIPMENT, NET	984.4				1,059.0				984.4	1,059.0	1,070.5
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									1,236.2	1,206.4	1,923.2
PROPERTY, PLANT AND EQUIPMENT, NET	961.0				1,012.7				961.0	1,012.7	998.1
Australia
Revenues from External Customers and Long-Lived Assets [Line Items]
PROPERTY, PLANT AND EQUIPMENT, NET	$ 23.4				$ 46.3				23.4	46.3	72.4
China [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									452.5	370.8	662.7
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									267.1	282.4	430.5
Other Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									$ 153.2	$ 153.7	$ 356.8