Supplementary Financial Statement Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Net sales	$ 1,746.6	$ 1,557.2	$ 3,405.5	$ 3,090.6
Non-US [Member]
Disaggregation of Revenue [Line Items]
Net sales	172.9	157.7	340.7	312.8
Carbon steel
Disaggregation of Revenue [Line Items]
Net sales	1,102.4	1,048.1	2,171.0	2,077.3
Stainless and electrical steel
Disaggregation of Revenue [Line Items]
Net sales	483.1	444.1	909.2	878.0
Tubular products, components and other
Disaggregation of Revenue [Line Items]
Net sales	161.1	65.0	325.3	135.3
Automotive
Disaggregation of Revenue [Line Items]
Net sales	1,108.7	1,007.7	2,227.8	2,037.2
Infrastructure and Manufacturing
Disaggregation of Revenue [Line Items]
Net sales	263.8	249.0	505.1	478.6
Distributors and Converters
Disaggregation of Revenue [Line Items]
Net sales	$ 374.1	$ 300.5	$ 672.6	$ 574.8