Related Parties (Summary Of Other Ownership Interests) (Detail)	Sep. 30, 2012	Sep. 30, 2011
Empire [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	79.00%	79.00%
Tilden [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	85.00%	85.00%
Bloom Lake [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	75.00%	75.00%
Cliffs Natural Resources [Member] | Empire [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	79.00%
Cliffs Natural Resources [Member] | Tilden [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	85.00%
Cliffs Natural Resources [Member] | Bloom Lake [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	75.00%
Arcelor Mittal [Member] | Empire [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	21.00%
Arcelor Mittal [Member] | Tilden [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	0.00%
Arcelor Mittal [Member] | Hibbing [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	62.30%
Arcelor Mittal [Member] | Bloom Lake [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	0.00%
U. S. Steel Canada [Member] | Empire [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	0.00%
U. S. Steel Canada [Member] | Tilden [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	15.00%
U. S. Steel Canada [Member] | Hibbing [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	14.70%
U. S. Steel Canada [Member] | Bloom Lake [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	0.00%
WISCO [Member] | Empire [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	0.00%
WISCO [Member] | Tilden [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	0.00%
WISCO [Member] | Hibbing [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	0.00%
WISCO [Member] | Bloom Lake [Member]
Related Party Transaction [Line Items]
Ownership interest, percent	25.00%
Other Liabilities [Member] | Hibbing [Member] | Cliffs Natural Resources [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	23.00%