Acquisitions (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2016 location
Acquisitions
Net sales	$ 2,450.1	$ 2,185.2	$ 7,344.6	$ 6,551.8
Allocation of the total purchase price of the acquisitions to the fair value of the assets acquired and liabilities assumed
Goodwill	1,834.0		1,834.0		$ 1,827.4
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		1.5
Accounts receivable	14.1		14.1
Inventories	66.6		66.6
Property, plant and equipment	62.2		62.2
Goodwill	104.7		104.7
Intangible assets subject to amortization	77.1		77.1
Intangible assets not subject to amortization	38.2		38.2
Other current and long-term assets	0.5		0.5
Total assets acquired	364.9		364.9
Current and long-term debt	6.1		6.1
Other current and long-term liabilities	7.3		7.3
Total liabilities assumed	13.4		13.4
Net assets acquired	$ 351.5		351.5
Alaska Steel
Acquisitions
Net sales			17.3
Best Manufacturing
Acquisitions
Net sales			16.8
Tubular Steel
Acquisitions
Number of locations of the acquiree entity | location						6
Net sales			$ 102.5