Acquisitions (Details) $ in Millions				3 Months Ended								12 Months Ended			21 Months Ended
	May. 16, 2014 USD ($)	Apr. 30, 2013 USD ($) item	Apr. 12, 2013 USD ($) item	Dec. 31, 2015 USD ($) item	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2014 USD ($)	Aug. 01, 2014 country item	Apr. 04, 2013 USD ($)	Dec. 31, 2012 USD ($)
Acquisitions
Net sales				$ 2,026.2	$ 2,286.2	$ 2,423.7	$ 2,614.4	$ 2,576.7	$ 2,705.1	$ 2,616.8	$ 2,553.0	$ 9,350.5	$ 10,451.6	$ 9,223.8
Proceeds from sale of business, net													26.2	11.9
Number of countries of the acquiree entity | item				12								12
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Goodwill				$ 1,724.8				1,736.4				$ 1,724.8	1,736.4	1,691.6	$ 1,736.4			$ 1,314.6
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount				$ 558.7								558.7
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								39.0					39.0	206.8	39.0
Summary purchase price allocation information for all acquisitions
Intangible assets not subject to amortization								39.0					39.0	206.8	39.0
Customer relationships
Summary purchase price allocation information for all acquisitions
Intangible assets acquired subject to amortization													$ 37.3	$ 135.3
Weighted average lives of identifiable intangible assets													13 years 7 months 6 days	12 years 6 months
Unsecured revolving credit facility
Acquisitions
Maximum borrowing capacity of the revolving credit facility			$ 1,500.0														$ 1,500.0
Term loan expiring April 4, 2018
Acquisitions
Face amount of debt, proceeds from which is used to fund transaction			500.0
Senior unsecured notes due April 15, 2023
Acquisitions
Face amount of debt, proceeds from which is used to fund transaction			500.0
NIS
Acquisitions
Net sales												19.9
AMS
Acquisitions
Net sales												257.6
Number of countries of the acquiree entity | country																4
AMS | Minimum
Acquisitions
Number of countries in which entity's customers operate | item																40
Travel Main
Acquisitions
Number of real estate properties of acquired entity which were leased by subsidiaries of reporting entity | item		18
Total transaction value		$ 78.9
Value of debt assumed		$ 43.8
Metals USA
Acquisitions
Net sales												1,540.0
Total transaction value			1,250.0
Value of debt assumed			$ 486.1
Number of service centers acquired | item			41
Purchase price			$ 766.8
Acquired debt not refinanced through proceeds from debt			12.3
Transaction costs														$ 11.4
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Cash			3.2
Accounts receivable			206.0
Inventories			379.5
Property, plant and equipment			242.6
Goodwill			382.7
Intangible assets subject to amortization			137.6
Intangible assets not subject to amortization			203.0
Other current and long-term assets			9.1
Total assets acquired			1,563.7
Deferred taxes			184.4
Current and long-term debt			486.1
Other current and long-term liabilities			126.4
Total liabilities assumed			796.9
Net assets acquired			766.8
Summary purchase price allocation information for all acquisitions
Intangible assets not subject to amortization			$ 203.0
Metals USA | Non-core roofing business | Disposal Group, Not Discontinued Operations
Acquisitions
Net sales													$ 9.6		25.4
Proceeds from sale of business, net	$ 26.2
Pre-tax loss on sale of business	1.1
Fox
Acquisitions
Net sales												21.6
Haskins Steel Co., Inc. (Haskins Steel)
Acquisitions
Net sales												$ 26.0
2014 acquisitions
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Cash								1.6					1.6		1.6
Accounts receivable								67.1					67.1		67.1
Inventories								89.2					89.2		89.2
Property, plant and equipment								23.4					23.4		23.4
Goodwill								51.3					51.3		51.3
Intangible assets subject to amortization								37.5					37.5		37.5
Intangible assets not subject to amortization								39.0					39.0		39.0
Other current and long-term assets								1.5					1.5		1.5
Total assets acquired								310.6					310.6		310.6
Deferred taxes								9.0					9.0		9.0
Current and long-term debt								39.2					39.2		39.2
Other current and long-term liabilities								53.1					53.1		53.1
Total liabilities assumed								101.3					101.3		101.3
Net assets acquired								209.3					209.3		209.3
Summary purchase price allocation information for all acquisitions
Intangible assets not subject to amortization								39.0					39.0		39.0
Tax deductible goodwill amount								$ 20.3					$ 20.3		$ 20.3
2013 acquisitions
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount														$ 107.7
Businesses Sold
Summary purchase price allocation information for all acquisitions
Tax deductible goodwill amount	$ 17.2