Supplementary Financial Statement Information Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Net sales	$ 1,680.5	$ 1,746.6	$ 3,378.2	$ 3,405.5
Non-US [Member]
Disaggregation of Revenue [Line Items]
Net sales	152.1	172.9	295.3	340.7
Carbon steel
Disaggregation of Revenue [Line Items]
Net sales	1,102.2	1,102.4	2,201.4	2,171.0
Stainless and electrical steel
Disaggregation of Revenue [Line Items]
Net sales	431.0	483.1	875.5	909.2
Tubular products, components and other
Disaggregation of Revenue [Line Items]
Net sales	147.3	161.1	301.3	325.3
Automotive
Disaggregation of Revenue [Line Items]
Net sales	1,081.7	1,108.7	2,172.6	2,227.8
Infrastructure and Manufacturing
Disaggregation of Revenue [Line Items]
Net sales	267.7	263.8	545.8	505.1
Distributors and Converters
Disaggregation of Revenue [Line Items]
Net sales	$ 331.1	$ 374.1	$ 659.8	$ 672.6