Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue Disaggregation
Revenue	$ 2,883.5	$ 2,988.9	$ 5,840.1	$ 5,746.0
Carbon steel
Revenue Disaggregation
Revenue	1,532.7	1,642.2	3,126.3	3,103.4
Aluminum
Revenue Disaggregation
Revenue	564.4	570.0	1,130.2	1,120.2
Stainless steel
Revenue Disaggregation
Revenue	407.2	428.7	813.7	831.9
Alloy
Revenue Disaggregation
Revenue	170.0	172.5	354.3	341.5
Toll processing and logistics
Revenue Disaggregation
Revenue	108.1	104.3	219.8	205.4
Other
Revenue Disaggregation
Revenue	$ 101.1	$ 71.2	$ 195.8	$ 143.6