Acquisitions (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) item	Dec. 31, 2021 USD ($) $ / shares	Dec. 31, 2020 USD ($) $ / shares
Acquisitions
Net sales	$ 17,025.0	$ 14,093.3	$ 8,811.9
Number of states in which the company operates metal service center network | item	40
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Goodwill	$ 2,105.9	2,107.6	1,935.2
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount	854.5
Trade names
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Intangible assets not subject to amortization	51.2
Production backlog
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Intangible assets subject to amortization	$ 23.8
Summary purchase price allocation information for all acquisitions
Weighted average lives of identifiable intangible assets	7 years 10 months 24 days
Customer relationships
Summary purchase price allocation information for all acquisitions
Intangible assets acquired subject to amortization	$ 92.3
Weighted average lives of identifiable intangible assets	10 years
2021 Acquisitions
Acquisitions
Net sales	$ 863.0
Transaction costs		7.7
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Cash	1.0
Accounts receivable	107.2
Inventories	134.4
Property, plant and equipment	33.6
Operating lease right-of-use assets	29.8
Goodwill	177.3
Intangible assets subject to amortization	116.3
Intangible assets not subject to amortization	51.2
Other current and long-term assets	4.0
Total assets acquired	654.8
Deferred taxes	48.6
Operating lease liabilities	24.6
Other current and long-term liabilities	141.3
Total liabilities assumed	214.5
Net assets acquired	440.3
Pro forma financial information
Non-recurring Inventory step-up amortization			21.8
Non-recurring excess renumeration payments			7.8
Non-recurring acquisition related costs			7.7
Pro forma net income attributable to Reliance after non-recurring items			$ 384.6
Pro forma diluted earnings per share adjusted for non-recurring items | $ / shares			$ 5.89
Pro forma:
Net sales		14,820.5	$ 9,345.4
Net income attributable to Reliance		$ 1,518.0	$ 356.6
Basic earnings per common share attributable to Reliance shareholders (in dollars per share) | $ / shares		$ 24.01	$ 5.54
Diluted earnings per common share attributable to Reliance shareholders (in dollars per share) | $ / shares		$ 23.60	$ 5.46
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount	$ 106.4