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Summary of Significant Accounting Policies - Revenue Recognition, Share-Based Compensation and Foreign Currencies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Revenue Recognition
Number of performance obligations from sales of metal products | item	1
Shipping and handling costs included in operating expenses	$ 404.1	$ 412.7	$ 372.3
Share-Based Compensation
Stock-based compensation expense	51.2	45.5	33.4
Foreign Currencies
Net gain (loss) resulting from foreign currency transactions	$ (4.1)	$ 0.6	$ (4.9)
Toll processing and logistics
Revenue Recognition
Number Of Days To Perform Services	1 day

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Cost incurred during the reporting period in transporting goods and services to customers. Includes freight-out cost.

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Amount before tax of foreign currency transaction realized and unrealized gain (loss) recognized in the income statement.

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