Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue Disaggregation
Revenue	$ 2,019.3	$ 2,883.5	$ 4,592.2	$ 5,840.1
Carbon steel
Revenue Disaggregation
Revenue	1,073.8	1,532.7	2,390.0	3,126.3
Aluminum
Revenue Disaggregation
Revenue	393.4	564.4	905.6	1,130.2
Stainless steel
Revenue Disaggregation
Revenue	336.4	407.2	741.3	813.7
Alloy
Revenue Disaggregation
Revenue	99.9	170.0	251.3	354.3
Toll processing and logistics
Revenue Disaggregation
Revenue	61.4	114.0	173.9	231.7
Other and eliminations
Revenue Disaggregation
Revenue	$ 54.4	$ 95.2	$ 130.1	$ 183.9