Revenues (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)
Revenue Disaggregation
Revenue	$ 2,974.5		$ 8,720.5
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		$ 2,450.1		$ 7,344.6
Carbon steel
Revenue Disaggregation
Revenue	1,637.6		$ 4,741.0
Carbon steel | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		1,319.5		3,936.8
Aluminum
Revenue Disaggregation
Revenue	559.9		1,680.1
Aluminum | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		475.3		1,446.2
Stainless steel
Revenue Disaggregation
Revenue	418.7		1,250.6
Stainless steel | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		343.3		1,038.6
Alloy
Revenue Disaggregation
Revenue	171.2		512.7
Alloy | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		149.3		440.4
Toll processing and logistics
Revenue Disaggregation
Revenue	106.5		$ 311.9
Number of days to perform services			1 day
Toll processing and logistics | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		85.8		258.8
Other and eliminations
Revenue Disaggregation
Revenue	$ 80.6		$ 224.2
Other and eliminations | Before adoption of ASU 2014-09
Revenue Disaggregation
Revenue		$ 76.9		$ 223.8