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Acquisitions - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jul. 02, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Business Acquisition [Line Items]
Revenue		$ 1,204.9	$ 1,057.1	$ 2,435.7		$ 1,998.4
Net income (loss)		16.4	$ 17.5	$ 45.9		$ 27.9
Central Steel & Wire Company [Member]
Business Acquisition [Line Items]
Effective date of acquisition				Jul. 02, 2018
Description of business acquisition				On July 2, 2018 (“the acquisition date”) JT Ryerson purchased Central Steel & Wire Company (“CS&W”). CS&W is a leading metal service center with locations across the Central and Eastern United States offering a wide selection of products and capabilities, with a commercial portfolio centered on bar, tube, plate, and steel products.
Business combination, fair value of consideration	$ 163.5
Business combination, fair value of accounts receivables acquired	80.0
Business combination, accounts receivables acquired, gross	81.8
Business combination, accounts receivables, expected uncollectible	1.8
Bargain purchase gain	70.0
Business combination, acquisition related fees					$ 1.6
Revenue		154.3		$ 326.5
Net income (loss)		$ (1.4)		$ 2.2
Central Steel & Wire Company [Member] \| Other Income and (Expense), Net [Member]
Business Acquisition [Line Items]
Bargain purchase gain					$ 70.0
Central Steel & Wire Company [Member] \| Trademark [Member]
Business Acquisition [Line Items]
Intangibles assets recognized from business acquisitions	$ 16.1
Business combination, useful life of intangible assets acquired	10 years
Fanello Industries [Member]
Business Acquisition [Line Items]
Effective date of acquisition				Apr. 02, 2018
Description of business acquisition				On April 2, 2018, Ryerson Holding acquired Fanello Industries, LLC (“Fanello”), a privately owned metal service company located in Lavonia, Georgia.