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OPERATING SEGMENT INFORMATION - Narrative (Details) | 12 Months Ended
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Dec. 31, 2020 EUR (€) employee facility country | Dec. 31, 2019 EUR (€) | Dec. 31, 2018 EUR (€)
Disclosure of operating segments [line items] |  |  |
Number of employees | employee | 12,000 |  |
Revenue | € 4,883,000,000 | € 5,907,000,000 | € 5,686,000,000
P&ARP |  |  |
Disclosure of operating segments [line items] |  |  |
Number of facilities | facility | 4 |  |
Number of countries in which entity operates | country | 3 |  |
Number of employees | employee | 3,900 |  |
Revenue | € 2,725,000,000 | 3,139,000,000 | 3,050,000,000
P&ARP | Group's largest customer |  |  |
Disclosure of operating segments [line items] |  |  |
Revenue | € 492,000,000 | 0 | 812,000,000
A&T |  |  |
Disclosure of operating segments [line items] |  |  |
Number of facilities | facility | 6 |  |
Number of countries in which entity operates | country | 3 |  |
Number of employees | employee | 3,300 |  |
Revenue | € 1,002,000,000 | 1,420,000,000 | 1,339,000,000
AS&I |  |  |
Disclosure of operating segments [line items] |  |  |
Number of facilities | facility | 19 |  |
Number of countries in which entity operates | country | 10 |  |
Number of employees | employee | 4,600 |  |
Revenue | € 1,156,000,000 | € 1,348,000,000 | € 1,287,000,000

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