INTANGIBLE ASSETS - Impairment testing of goodwill (Details) - CNY (¥) ¥ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Groups of CGUs according to operating segments
Goodwill	¥ 3,510,000
Pre-tax cash flow projections period approved by management	5 years
Estimated growth rate for cash flows beyond approved period	2.00%	2.00%
Pre-tax rate that reflects specific risks related to CGUs and groups of CGUs as discount rate	12.62%	12.62%
Impairment of goodwill	¥ 0	¥ 0
Alumina
Groups of CGUs according to operating segments
Goodwill	1,368,331	1,369,366
Alumina \| Guangxi Branch
Groups of CGUs according to operating segments
Goodwill	189,419	189,419
Alumina \| PT. Nusapati Prima
Groups of CGUs according to operating segments
Goodwill	14,963	15,998
Alumina \| Shanxi Huaxing
Groups of CGUs according to operating segments
Goodwill	1,163,949	1,163,949
Primary aluminum
Groups of CGUs according to operating segments
Goodwill	2,141,526	2,141,526
Primary aluminum \| Qinghai Branch
Groups of CGUs according to operating segments
Goodwill	217,267	217,267
Primary aluminum \| Lanzhou Aluminum Co,. Ltd
Groups of CGUs according to operating segments
Goodwill	¥ 1,924,259	¥ 1,924,259

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The discount rate applied to cash flow projections for a cash-generating unit (group of units). [Refer: Cash-generating units [member]]

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The growth rate used to extrapolate cash flow projections beyond the period covered by the most recent budgets/forecasts for a cash-generating unit (group of units). [Refer: Cash-generating units [member]]

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The amount of assets representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognised. [Refer: Business combinations [member]]

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The amount of impairment loss recognised in profit or loss for goodwill. [Refer: Impairment loss recognised in profit or loss; Goodwill]

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