Acquisitions (Details) $ in Millions	2 Months Ended	3 Months Ended								5 Months Ended	10 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($) location	Dec. 31, 2018 USD ($) location	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) location	Dec. 31, 2018 USD ($) location	Dec. 31, 2018 USD ($) location	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 01, 2018 location	Oct. 23, 2018	Oct. 22, 2018	Jan. 01, 2016 location	Dec. 31, 2015 USD ($)
Acquisitions
Net sales		$ 2,814.0	$ 2,974.5	$ 2,988.9	$ 2,757.1	$ 2,376.4	$ 2,450.1	$ 2,475.2	$ 2,419.3			$ 11,534.5	$ 9,721.0	$ 8,613.4
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Goodwill value	$ 1,870.8	1,870.8				1,842.6				$ 1,870.8	$ 1,870.8	1,870.8	1,842.6	1,827.4					$ 1,724.8
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount	685.4	685.4								685.4	685.4	685.4
Summary Information for Equity Method Transaction
Consideration paid for additional ownership percentage												29.0
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	18.3				3.7				18.3	18.3	18.3	3.7	38.2
Summary purchase price allocation information for all acquisitions
Intangible assets not subject to amortization	18.3	$ 18.3				$ 3.7				$ 18.3	$ 18.3	18.3	3.7	38.2
Customer relationships
Summary purchase price allocation information for all acquisitions
Intangible assets acquired subject to amortization												$ 24.8	$ 3.7	$ 76.8
Weighted average lives of identifiable intangible assets												10 years	10 years	15 years 6 months
All Metals
Acquisitions
Net sales	$ 4.2
Locations acquiree provides logistics services from | location															6
Acero Prime S. de R.L. de C.V.
Acquisitions
Net sales												$ 41.0
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	4								4	4	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										$ 11.9
DuBose
Acquisitions
Net sales											$ 26.9
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	$ 2.4								2.4	2.4	2.4
Accounts receivable	13.1	13.1								13.1	13.1	13.1
Inventories	10.0	10.0								10.0	10.0	10.0
Property, plant and equipment	20.2	20.2								20.2	20.2	20.2
Goodwill value	33.8	33.8								33.8	33.8	33.8
Intangible assets subject to amortization	25.0	25.0								25.0	25.0	25.0
Intangible assets not subject to amortization	18.3	18.3								18.3	18.3	18.3
Other current and long-term assets	1.1	1.1								1.1	1.1	1.1
Total assets acquired	123.9	123.9								123.9	123.9	123.9
Current and long-term debt	25.9	25.9								25.9	25.9	25.9
Deferred taxes	7.8	7.8								7.8	7.8	7.8
Other current and long-term liabilities	9.2	9.2								9.2	9.2	9.2
Total liabilities assumed	42.9	42.9								42.9	42.9	42.9
Net assets acquired	81.0	81.0								81.0	81.0	81.0
Summary purchase price allocation information for all acquisitions
Intangible assets not subject to amortization	18.3	18.3								18.3	18.3	18.3
Ferguson
Acquisitions
Sales since acquisition date												39.4
Alaska Steel
Acquisitions
Net sales												26.7
Best Manufacturing
Acquisitions
Net sales												26.3
Tubular Steel
Acquisitions
Net sales												174.4
Number of locations of the acquiree entity | location																		6
2018 Acquisitions
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount	21.3	21.3								21.3	21.3	21.3
2016 Acquisitions
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Cash														$ 1.5
Accounts receivable														14.1
Inventories														66.6
Property, plant and equipment														62.2
Goodwill value														104.7
Intangible assets subject to amortization														77.1
Intangible assets not subject to amortization														38.2
Other current and long-term assets														0.5
Total assets acquired														364.9
Current and long-term debt														6.1
Other current and long-term liabilities														7.3
Total liabilities assumed														13.4
Net assets acquired														351.5
Summary purchase price allocation information for all acquisitions
Intangible assets not subject to amortization														38.2
Tax deductible goodwill amount														104.7
2018, 2017 and 2016 Acquisitions
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount	$ 21.3	$ 21.3								$ 21.3	$ 21.3	$ 21.3		$ 104.7