Basis Of Presentation And Significant Accounting Policies (Schedule Of Subsidiaries) (Detail)	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Northshore [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	100.00%
Location	Minnesota
Operation	Iron Ore
United Taconite [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	100.00%
Location	Minnesota
Operation	Iron Ore
Wabush [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	100.00%
Location	Labrador/Quebec, Canada
Operation	Iron Ore
Bloom Lake [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	75.00%	75.00%
Location	Quebec, Canada
Operation	Iron Ore
Tilden [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	85.00%	85.00%
Location	Michigan
Operation	Iron Ore
Empire [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	79.00%	79.00%
Location	Michigan
Operation	Iron Ore
Koolyanobbing [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	100.00%
Location	Western Australia
Operation	Iron Ore
Pinnacle [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	100.00%
Location	West Virginia
Operation	Coal
Oak Grove [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	100.00%
Location	Alabama
Operation	Coal
CLCC [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	100.00%
Location	West Virginia
Operation	Coal