Business, Product and Geographical Area Information and Concentration of Risk, Supply Information (Details) - Supplier Concentration Risk - Aluminum	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Top five major suppliers
Concentration Risk
Percentage of total primary aluminum supply (percent)	83.00%	86.00%	82.00%	86.00%
Largest supplier
Concentration Risk
Percentage of total primary aluminum supply (percent)	33.00%	35.00%	36.00%	36.00%
Second and third largest suppliers
Concentration Risk
Percentage of total primary aluminum supply (percent)	36.00%	35.00%	32.00%	33.00%

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.

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