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Business Segments - Major Customers (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Revenue, Major Customer [Line Items]
Major customer, percentage revenue, net									10.00%
Net sales	$ 486,900,000	$ 481,800,000	$ 470,000,000	$ 454,500,000	$ 433,900,000	$ 400,600,000	$ 388,800,000	$ 365,800,000	$ 1,893,200,000	$ 1,589,100,000	[1]	$ 1,319,100,000
Glencore [Member]
Revenue, Major Customer [Line Items]
Net sales									1,204,500,000	1,198,100,000		1,178,600,000
Southwire [Member]
Revenue, Major Customer [Line Items]
Net sales									$ 222,400	$ 77,200		$ 0
[1] As adjusted due to the adoption of ASU 2017-07 Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost

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