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BORROWINGS - Borrowings by Nature (Details) |  |  |  |  | 6 Months Ended | 12 Months Ended |  |  |  |  |
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Jun. 30, 2020 EUR (€) | May 13, 2020 EUR (€) | Aug. 08, 2019 EUR (€) | Mar. 28, 2018 EUR (€) | Jun. 30, 2020 EUR (€) | Dec. 31, 2020 EUR (€) | Dec. 31, 2019 EUR (€) | Dec. 31, 2018 EUR (€) | Dec. 31, 2020 USD ($) | Dec. 31, 2020 CHF (SFr) | Jul. 15, 2020 EUR (€) credit_facility | Jun. 30, 2020 USD ($) | May 22, 2020 EUR (€) | Apr. 24, 2020 EUR (€)
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal value |  |  |  |  |  | € 2,386,000,000 |  |  |  |  |  |  |  |
Gross lease liabilities |  |  |  |  |  | 194,000,000 |  |  |  |  |  |  |  |
Arrangement fees |  |  |  |  |  | (27,000,000) |  |  |  |  |  |  |  |
Accrued interests |  |  |  |  |  | 32,000,000 |  |  |  |  |  |  |  |
Accrued interest lease liabilities |  |  |  |  |  | 1,000,000 |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 2,391,000,000 | € 2,361,000,000 |  |  |  |  |  |  |
Leases |  |  |  |  |  | 195,000,000 | 188,000,000 |  |  |  |  |  |  |
Non-current borrowings |  |  |  |  |  | 2,299,000,000 | 2,160,000,000 |  |  |  |  |  |  |
Current borrowings |  |  |  |  |  | 92,000,000 | 201,000,000 |  |  |  |  |  |  |
Repurchase of senior notes |  |  |  |  |  | 200,000,000 | 100,000,000 | € 0 |  |  |  |  |  |
Secured Pan-U.S. ABL (due 2022) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 0 | 127,000,000 |  |  |  |  |  |  |
Secured U.S. DDTL (due 2022) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 0 | 0 |  |  |  |  |  |  | € 166,000,000
Net orderly liquidation value of eligible equipment percentage |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00%
Secured French PGE French Facility (due 206) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal value |  |  |  |  |  | 180,000,000 |  |  | $ 180,000,000 |  |  |  |  |
Arrangement fees |  |  |  |  |  | 0 |  |  |  |  |  |  |  |
Accrued interests |  |  |  |  |  | 0 |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | € 180,000,000 | 0 |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | French Government |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  | € 180,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Percentage of borrowings guaranteed |  | 80.00% |  |  |  |  |  |  |  |  |  |  |  |
Extension period |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |
Percentage of borrowing costs |  | 0.50% |  |  |  |  |  |  |  |  |  |  |  |
Draw amortization period |  | 2 years |  |  |  |  |  |  |  |  |  |  |  |
Effective rate |  | 2.50% |  |  |  |  |  |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | French Government | First year |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of borrowing costs |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | French Government | Second year |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of borrowing costs |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | French Government | Third year |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of borrowing costs |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | French Government | Fourth year |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of borrowing costs |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | French Government | Fifth year |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of borrowing costs |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | French Government | Sixth year |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of borrowing costs |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | French Government | EURIBOR |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Extension period |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |
Interest rate floor |  | 0.00% |  |  |  |  |  |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | French Government | EURIBOR | For the first six months |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate margin |  | 1.30% |  |  |  |  |  |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | French Government | EURIBOR | Annually thereafter |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate margin |  | 0.50% |  |  |  |  |  |  |  |  |  |  |  |
Secured French PGE French Facility (due 206) | BPI France | French Government |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  | € 30,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Secured German Facility (due 2022) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal rate |  |  |  |  |  | 2.00% |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | € 0 | 0 |  |  |  |  |  |  |
Secured Inventory Facility (due 2021) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 0 | 0 |  |  |  |  |  |  |
Senior Unsecured Notes due 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal value |  |  |  |  |  | € 326,000,000 |  |  | 400,000,000 |  |  |  |  |
Nominal rate |  |  |  |  |  | 5.75% |  |  |  |  |  |  |  |
Arrangement fees |  |  |  |  |  | € (3,000,000) |  |  |  |  |  |  |  |
Accrued interests |  |  |  |  |  | 2,000,000 |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 325,000,000 | 355,000,000 |  |  |  |  |  |  |
Senior Unsecured Notes due 2021 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal value |  |  |  |  |  | € 0 |  |  |  |  |  |  |  |
Nominal rate |  |  |  |  | 4.625% | 4.625% |  |  |  |  |  |  |  |
Arrangement fees |  |  |  |  |  | € 0 |  |  |  |  |  |  |  |
Accrued interests |  |  |  |  |  | 0 |  |  |  |  |  |  |  |
Borrowings |  |  | € 300,000,000 |  |  | 0 | 200,000,000 |  |  |  |  |  |  |
Repurchase of senior notes | € 200,000,000 |  | € 100,000,000 |  |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes due 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal value |  |  |  |  |  | € 530,000,000 |  |  | 650,000,000 |  |  |  |  |
Nominal rate |  |  |  |  |  | 6.625% |  |  |  |  |  |  |  |
Arrangement fees |  |  |  |  |  | € (8,000,000) |  |  |  |  |  |  |  |
Accrued interests |  |  |  |  |  | 12,000,000 |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 534,000,000 | 582,000,000 |  |  |  |  |  |  |
Senior Unsecured Notes due 2026 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal value |  |  |  |  |  | € 407,000,000 |  |  | 500,000,000 |  |  |  |  |
Nominal rate |  |  |  |  |  | 5.875% |  |  |  |  |  |  |  |
Arrangement fees |  |  |  |  |  | € (5,000,000) |  |  |  |  |  |  |  |
Accrued interests |  |  |  |  |  | 9,000,000 |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 411,000,000 | 449,000,000 |  |  |  |  |  |  |
Senior Unsecured Note due 2026 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal value |  |  |  |  |  | € 400,000,000 |  |  |  |  |  |  |  |
Nominal rate |  |  |  |  |  | 4.25% |  |  |  |  |  |  |  |
Arrangement fees |  |  |  |  |  | € (5,000,000) |  |  |  |  |  |  |  |
Accrued interests |  |  |  |  |  | 6,000,000 |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 401,000,000 | 400,000,000 |  |  |  |  |  |  |
Senior Unscured Notes due 2028 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal value |  |  |  |  |  | € 265,000,000 |  |  | $ 325,000,000 |  |  | $ 325,000,000 |  |
Nominal rate |  |  |  |  | 5.625% | 5.625% |  |  |  |  |  |  |  |
Arrangement fees | € (6,000,000) |  |  |  | € (6,000,000) | € (6,000,000) |  |  |  |  |  |  |  |
Accrued interests |  |  |  |  |  | 1,000,000 |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 260,000,000 | 0 |  |  |  |  |  |  |
Unsecured Revolving Credit Facility (due 2021) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 0 | 0 |  |  |  |  |  |  |
Unsecured Revolving Credit Facility (due 2021) | BPI France |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowing capacity |  |  |  | € 10,000,000 |  | 3,000,000 |  |  |  |  |  |  |  |
Unsecured Swiss Facility (due 2025) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal value |  |  |  |  |  | € 18,000,000 |  |  |  | SFr 20,000,000 |  |  |  |
Nominal rate |  |  |  |  |  | 1.18% |  |  |  |  |  |  |  |
Arrangement fees |  |  |  |  |  | € 0 |  |  |  |  |  |  |  |
Accrued interests |  |  |  |  |  | 0 |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | € 18,000,000 | 0 |  |  |  |  |  |  |
Unsecured Swiss Facility (due 2025) | Swiss Government |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  |  |  |  |  |  |  |  | € 20,000,000 |
Percentage of borrowings guaranteed |  |  |  |  |  |  |  |  |  |  |  |  | 85.00% |
Unsecured German Facilities (due 2022) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal rate |  |  |  |  |  | 2.12% |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | € 0 | 0 |  |  |  |  |  |  |
Other loans |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nominal value |  |  |  |  |  | 66,000,000 |  |  |  |  |  |  |  |
Arrangement fees |  |  |  |  |  | 0 |  |  |  |  |  |  |  |
Accrued interests |  |  |  |  |  | 1,000,000 |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | 67,000,000 | € 60,000,000 |  |  |  |  |  |  |
Secured and Unsecured German Credit Facilities due 2022 | German State |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of borrowings guaranteed |  |  |  |  |  |  |  |  |  |  | 80.00% |  |  |
Number of credit facilities | credit_facility |  |  |  |  |  |  |  |  |  |  | 2 |  |  |
Secured and Unsecured German Credit Facility due 2022 One | German State |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  |  |  |  |  |  | € 25,000,000 |  |  |
Secured and Unsecured German Facility due 2022 Two | German State |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  |  |  |  |  |  | € 25,000,000 |  |  |
Financing Arrangements |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | € 44,000,000 |  |  |  |  |  |  |  |

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