Long-Term Debt - Foreign Debt - Additional Information (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Foreign debt	$ 23.8	$ 19.2
Foreign Debt [Member]
Debt Instrument [Line Items]
Letters of credit issued by our foreign subsidiaries	1.0	1.0
Foreign Debt [Member] | Ryerson China [Member]
Debt Instrument [Line Items]
Available credit facility	21.0	26.0
Foreign Debt [Member] | Owed to Banks [Member] | Ryerson China [Member]
Debt Instrument [Line Items]
Foreign debt	$ 23.8	$ 19.2
Weighted average interest rate	4.10%	4.40%