OPERATING SEGMENT INFORMATION - Narrative (Details)	12 Months Ended
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