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Business, Product and Geographical Area Information and Concentration of Risk, Supply Information (Details) - Supplier Concentration Risk - Aluminum | 3 Months Ended | 6 Months Ended
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Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Top five major suppliers |  |  |  |
Concentration Risk |  |  |  |
Percentage of total primary aluminum supply (percent) | 74.00% | 83.00% | 72.00% | 82.00%
Largest supplier |  |  |  |
Concentration Risk |  |  |  |
Percentage of total primary aluminum supply (percent) | 21.00% | 35.00% | 21.00% | 37.00%
Second and third largest suppliers |  |  |  |
Concentration Risk |  |  |  |
Percentage of total primary aluminum supply (percent) | 33.00% | 34.00% | 31.00% | 30.00%

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