Revenues (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Revenue Disaggregation
Revenue	$ 2,572.9	$ 2,956.6
Carbon steel
Revenue Disaggregation
Revenue	1,316.2	1,593.6
Aluminum
Revenue Disaggregation
Revenue	512.2	565.8
Stainless steel
Revenue Disaggregation
Revenue	404.9	406.5
Alloy
Revenue Disaggregation
Revenue	151.4	184.3
Toll processing and logistics
Revenue Disaggregation
Revenue	112.5	111.7
Other and eliminations
Revenue Disaggregation
Revenue	$ 75.7	$ 94.7

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration