Long-Term Debt - Foreign Debt - Additional Information (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Foreign debt	$ 12.7	$ 13.2
Foreign Debt [Member]
Debt Instrument [Line Items]
Available credit facility	34.0	32.0
Letters of credit issued by our foreign subsidiaries	4.0	3.0
Foreign Debt [Member] | Owed to Banks [Member] | Ryerson China [Member]
Debt Instrument [Line Items]
Foreign debt	$ 12.7	$ 13.2
Weighted average interest rate	4.20%	4.30%