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Restricted Cash - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Restricted Cash and Cash Equivalents Items [Line Items]      
Restricted cash $ 4.0 $ 3.9 $ 5.3
Proceeds from Sale of Property Plant and Equipment
     
Restricted Cash and Cash Equivalents Items [Line Items]      
Restricted cash   0 0.9
Letter of Credit
     
Restricted Cash and Cash Equivalents Items [Line Items]      
Restricted cash   3.8 4.1
Letter of Credit | Ryerson China
     
Restricted Cash and Cash Equivalents Items [Line Items]      
Restricted cash   $ 0.1 $ 0.3