Supplementary Financial Statement Information Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Net sales	$ 1,535.5	$ 1,735.6	$ 4,913.7	$ 5,141.1
Non-US [Member]
Disaggregation of Revenue [Line Items]
Net sales	144.3	149.6	439.6	490.3
Carbon steel
Disaggregation of Revenue [Line Items]
Net sales	1,000.9	1,132.6	3,202.3	3,303.6
Stainless and electrical steel
Disaggregation of Revenue [Line Items]
Net sales	389.3	454.3	1,264.8	1,363.5
Tubular products, components and other
Disaggregation of Revenue [Line Items]
Net sales	145.3	148.7	446.6	474.0
Automotive
Disaggregation of Revenue [Line Items]
Net sales	1,040.0	1,036.7	3,212.6	3,264.5
Infrastructure and Manufacturing
Disaggregation of Revenue [Line Items]
Net sales	239.8	277.1	785.6	782.2
Distributors and Converters
Disaggregation of Revenue [Line Items]
Net sales	$ 255.7	$ 421.8	$ 915.5	$ 1,094.4