Acquisitions (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) item	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($) $ / shares
Acquisitions
Net sales	$ 14,805.9	$ 17,025.0	$ 14,093.3
Number of states in which the company operates metal service center network | item	40
Goodwill	$ 2,111.1	$ 2,105.9	2,107.6
2021 Acquisitions
Acquisitions
Net sales	$ 722.1
Transaction costs			7.7
Pro forma:
Net sales			14,820.5
Net income attributable to Reliance			$ 1,518.0
Basic earnings per common share attributable to Reliance shareholders (in dollars per share) | $ / shares			$ 24.01
Diluted earnings per common share attributable to Reliance shareholders (in dollars per share) | $ / shares			$ 23.60