Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue Disaggregation
Revenue	$ 2,447.8	$ 2,685.9	$ 2,883.5	$ 2,956.6	$ 2,814.0	$ 2,974.5	$ 2,988.9	$ 2,757.1	$ 10,973.8	$ 11,534.5	$ 9,721.0
Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue											9,721.0
Carbon steel
Revenue Disaggregation
Revenue									5,792.9	6,285.8
Carbon steel | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue											5,189.6
Aluminum
Revenue Disaggregation
Revenue									2,151.9	2,210.9
Aluminum | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue											1,916.9
Stainless steel
Revenue Disaggregation
Revenue									1,578.0	1,636.1
Stainless steel | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue											1,386.3
Alloy
Revenue Disaggregation
Revenue									652.1	680.7
Alloy | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue											587.8
Toll processing and logistics
Revenue Disaggregation
Revenue									450.7	415.3
Toll processing and logistics | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue											362.1
Other
Revenue Disaggregation
Revenue									$ 348.2	$ 305.7
Other | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue											$ 278.3