Revenues (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($)
Revenue Disaggregation
Revenue	$ 2,757.1
Revenue from sales of metal products (as a percent)	94.00%
Number of performance obligations from sales of metal products | item	1
Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		$ 2,419.3
Carbon steel
Revenue Disaggregation
Revenue	$ 1,461.2
Carbon steel | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		1,285.5
Aluminum
Revenue Disaggregation
Revenue	550.2
Aluminum | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		483.1
Stainless steel
Revenue Disaggregation
Revenue	403.2
Stainless steel | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		347.3
Alloy
Revenue Disaggregation
Revenue	169.0
Alloy | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		145.3
Toll processing and logistics
Revenue Disaggregation
Revenue	$ 101.1
Number of days to perform services	1 day
Toll processing and logistics | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		87.2
Other and eliminations
Revenue Disaggregation
Revenue	$ 72.4
Other and eliminations | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		$ 70.9