Business, Product and Geographical Area Information and Concentration of Risk, Supply Information (Details) - Supplier Concentration Risk - Aluminum	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Top five major suppliers
Concentration Risk
Percentage of total primary aluminum supply (percent)	71.00%	81.00%
Largest supplier
Concentration Risk
Percentage of total primary aluminum supply (percent)	20.00%	39.00%
Second and third largest suppliers
Concentration Risk
Percentage of total primary aluminum supply (percent)	28.00%	27.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Publisher FASB
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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