Acquisitions (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) location	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Nov. 01, 2018 location	Oct. 23, 2018	Oct. 22, 2018	Dec. 31, 2016 USD ($)
Acquisitions
Net sales	$ 10,973.8	$ 11,534.5	$ 9,721.0
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Goodwill	2,003.8	1,870.8	1,842.6				$ 1,827.4
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount	817.4
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		18.3	3.7
Summary purchase price allocation information for all acquisitions
Intangible assets not subject to amortization		18.3	3.7
Customer relationships
Summary purchase price allocation information for all acquisitions
Intangible assets acquired subject to amortization		$ 24.8	$ 3.7
Weighted average lives of identifiable intangible assets		10 years	10 years
Fry Steel
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Cash	17.1
Accounts receivable	5.7
Inventories	38.8
Property, plant and equipment	6.4
Goodwill	130.8
Other current and long-term assets	0.2
Total assets acquired	199.0
Other current and long-term liabilities	5.1
Total liabilities assumed	5.1
Net assets acquired	193.9
All Metals
Acquisitions
Net sales	29.7
Locations acquiree provides logistics services from | location				6
Acero Prime S. de R.L. de C.V.
Acquisitions
Net sales	$ 44.8
Ownership interest acquired (as a percent)					40.00%
Ownership interest prior to acquisition (as a percent)						60.00%
Total ownership percentage (as a percent)					100.00%
Number of locations of the acquiree entity | location	4
Summary Information for Equity Method Transaction
Consideration paid for additional ownership percentage		$ 29.0
Decrease in noncontrolling interest		19.7
Decrease in Reliance Stockholder's equity		9.3
KMS
Acquisitions
Net sales	$ 32.6
DuBose
Acquisitions
Net sales	42.9
All Metals, DuBose and KMS
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Cash		2.4
Accounts receivable		13.1
Inventories		10.0
Property, plant and equipment		20.1
Goodwill		32.9
Intangible assets subject to amortization		25.0
Intangible assets not subject to amortization		18.3
Other current and long-term assets		1.1
Total assets acquired		122.9
Current and long-term debt		25.9
Deferred taxes		5.4
Other current and long-term liabilities		9.6
Total liabilities assumed		40.9
Net assets acquired		82.0
Summary purchase price allocation information for all acquisitions
Intangible assets not subject to amortization		$ 18.3
Ferguson
Acquisitions
Sales since acquisition date	37.2
2019 Acquisitions
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount	$ 131.0