Supplementary Financial Statement Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Net sales	$ 1,658.9	$ 1,533.4
Non-US [Member]
Disaggregation of Revenue [Line Items]
Net sales	167.8	155.1
Carbon steel
Disaggregation of Revenue [Line Items]
Net sales	1,068.6	1,029.2
Stainless and electrical steel
Disaggregation of Revenue [Line Items]
Net sales	426.1	433.9
Tubular products, components and other
Disaggregation of Revenue [Line Items]
Net sales	164.2	70.3
Automotive
Disaggregation of Revenue [Line Items]
Net sales	1,119.1	1,029.5
Infrastructure and Manufacturing
Disaggregation of Revenue [Line Items]
Net sales	241.3	229.6
Distributors and Converters
Disaggregation of Revenue [Line Items]
Net sales	$ 298.5	$ 274.3