Employee Benefits - Additional Information (Detail) $ in Millions							1 Months Ended	2 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 24, 2021 USD ($)	Dec. 01, 2020 USD ($)	Nov. 30, 2020	Sep. 30, 2020 USD ($)	Sep. 28, 2020 USD ($)	Nov. 30, 2020 USD ($)	Nov. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2022	Dec. 31, 2021 USD ($) Employee Location	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2014	Dec. 31, 2012 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan Expense												$ 8.9	$ 4.3	$ 8.8
Payment price for purchased annuities on behalf of participants		$ 206.6	$ 19.7			$ 95.2
Pension settlement charge due to annuitization	$ 98.3				$ 52.5		$ 10.5
Discount rate	2.80%			2.42%	2.59%		2.59%		2.42%	3.15%
Expected long-term rate of return on pension assets	4.35%			5.15%	5.25%		5.25%		5.05%	5.75%
Other deferred employee benefit plans												$ 14.3	15.5
Defined benefit plan return on plan asset gain loss recognized period												4 years
Defined Benefit Plan, Fair Value Of Plan Assets												$ 333.8	520.3
Contribution to the pension plan fund												23.7	7.1	25.7
Anticipated minimum required pension contribution funding												$ 5.8
Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on pension assets												1.80%
Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on pension assets												4.85%
Change in Increase (Decrease) in Discount Rate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan benefit obligation period increase (decrease)												$ 18.2	62.4
Decrease Due to Updated Mortality Tables [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan benefit obligation period increase (decrease)												9.8	5.7
Decrease Due To Salary Scale Updates [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan benefit obligation period increase (decrease)													1.2
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, benefit obligation												429.6	674.7	771.8
Defined Benefit Plan, Fair Value Of Plan Assets												333.8	520.3	631.8
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, benefit obligation												$ 59.3	$ 68.3	$ 69.2
U.S. Plans [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Annual rate of increase in per capita cost of covered health care benefits, ultimate rate															4.50%
Annual rate of increase in per capita cost of covered health care benefits, date															2030
U.S. Plans [Member] | Other Postretirement Benefits [Member] | Under 65 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Annual rate of increase in per capita cost of covered health care benefits, rate												6.10%	6.20%		6.20%
Annual rate of increase in per capita cost of covered health care benefits, ultimate rate												4.50%	4.50%
Annual rate of increase in per capita cost of covered health care benefits, date												2030	2030
U.S. Plans [Member] | Other Postretirement Benefits [Member] | Over 65 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Annual rate of increase in per capita cost of covered health care benefits, rate												6.30%	6.50%		6.50%
Annual rate of increase in per capita cost of covered health care benefits, ultimate rate												4.50%	4.50%
Annual rate of increase in per capita cost of covered health care benefits, date												2030	2030
Canadian Plans [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, benefit obligation												$ 44.5	$ 48.3
Defined Benefit Plan, Fair Value Of Plan Assets												$ 44.5	$ 45.1
Canadian Plans [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate												2.19%	2.97%	3.53%
Defined benefit plan, benefit obligation												$ 13.5	$ 14.9
Annual rate of increase in per capita cost of covered health care benefits, rate												6.95%	6.95%		6.95%
Annual rate of increase in per capita cost of covered health care benefits, ultimate rate												4.50%	4.50%		4.50%
Annual rate of increase in per capita cost of covered health care benefits, date												2033	2033		2033
Central Steel and Wire Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Benefit Obligation, Benefits Paid												$ 7.5	$ 14.3	$ 14.3
Settlement loss												$ 0.4	$ 2.9	$ 1.6
Central Steel and Wire Retirement Plan [Member] | U.S. Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on pension assets												2.05%	3.20%	4.00%
Central Steel and Wire Retirement Plan [Member] | U.S. Plans [Member] | Scenario Forecast [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on pension assets											1.80%
Ryerson Pension Plan [Member] | U.S. Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on pension assets								5.25%	5.05%	5.75%		4.35%	5.15%	6.35%
Ryerson Pension Plan [Member] | U.S. Plans [Member] | Scenario Forecast [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on pension assets											4.85%
Ryerson Salaried Plan [Member] | Canadian Plans [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate												2.32%	3.00%	3.56%
Expected long-term rate of return on pension assets												4.25%	4.75%	5.25%
Defined benefit plan assets, percentage												80.00%
Ryerson Salaried Plan [Member] | Canadian Plans [Member] | Scenario Forecast [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on pension assets											4.25%
Ryerson Bargaining Unit Plan [Member] | Canadian Plans [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate												2.34%	3.01%	3.58%
Expected long-term rate of return on pension assets												1.75%	3.00%	3.50%
Defined benefit plan assets, percentage												20.00%
Ryerson Bargaining Unit Plan [Member] | Canadian Plans [Member] | Scenario Forecast [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on pension assets											2.25%
Multiemployer Pension and Other Postretirement Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer plans description												Ryerson participate in two multiemployer pension plans covering 39 employees at 4 locations.
Multiemployer pension plan covered employees | Employee												39
Multiemployer pension plan covered locations | Location												4
Multiemployer plan contributions												$ 0.4	$ 0.3	$ 0.4
Pension withdrawal liability												$ 0.4	$ 0.4			$ 1.0
Period of Withdrawal pension liability Payment												25 years
Multiemployer Pension and Other Postretirement Plans [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of contributions to the plans												5.00%