Acquisitions (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($) $ / shares	Apr. 01, 2024 item	Feb. 01, 2024 item	Dec. 31, 2022 USD ($)
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Goodwill	$ 2,161.8	$ 2,111.1			$ 2,105.9
Cooksey Iron & Metal Company
Acquisitions
Number of locations of the acquiree entity | item				3
American Alloy Steel, Inc
Acquisitions
Number of locations of the acquiree entity | item			5
2024 Acquisitions
Acquisitions
Sales since acquisition date	286.2
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Cash	5.6
Accounts receivable	44.9
Inventories	109.9
Prepaid expenses and other current assets	1.0
Property, plant and equipment	107.5
Operating lease right-of-use assets	19.2
Goodwill	56.0
Intangible assets subject to amortization	39.5
Intangible assets not subject to amortization	41.4
Total assets acquired	425.0
Deferred income taxes	6.7
Operating lease liabilities	15.1
Other current and long-term liabilities	32.7
Total liabilities assumed	54.5
Noncontrolling interest	0.3
Net assets acquired	370.2
Pro forma:
Net sales	13,943.5	15,312.8
Net income attributable to Reliance		$ 1,361.7
Basic earnings per share attributable to Reliance shareholders (in dollars per share) | $ / shares		$ 23.35
Diluted earnings per share attributable to Reliance shareholders (in dollars per share) | $ / shares		$ 23.07
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount	31.4
2024 Acquisitions | 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	41.4
2024 Acquisitions | Customer relationships
Summary purchase price allocation information for all acquisitions
Intangible assets acquired subject to amortization	$ 39.3
Weighted average lives of identifiable intangible assets	13 years 1 month 6 days
2024 Acquisitions | Non-compete agreements
Summary purchase price allocation information for all acquisitions
Intangible assets acquired subject to amortization	$ 0.2
Weighted average lives of identifiable intangible assets	5 years