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PENSIONS AND OTHER POST-EMPLOYMENT BENEFIT OBLIGATIONS - Movement in Net Defined Benefit Obligations (Details) € in Millions, $ in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2020 EUR (€) | Sep. 30, 2020 EUR (€) | Sep. 30, 2019 EUR (€) | Dec. 31, 2020 EUR (€) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 EUR (€)
Disclosure of net defined benefit liability (asset) [line items] |  |  |  |  |  |
Beginning balance |  |  |  | € 670 | $ 670 | € 610
Current service cost |  |  |  | 28 |  | 24
Interest cost / (income) |  |  |  | 11 |  | 16
Past service cost |  |  | € (36) | 2 |  | (1)
Immediate recognition of gains arising over the year |  |  |  | 2 |  | 2
Administration expenses |  |  |  | 2 |  | 2
Remeasurements due to: |  |  |  |  |  |
—actual return less interest on plan assets |  |  |  | (28) |  | (54)
—changes in financial assumptions |  |  |  | 67 |  | 126
—changes in demographic assumptions |  |  |  | (10) |  | (4)
—experience losses |  |  |  | (3) |  | (9)
Effects of changes in foreign exchange rates |  |  |  | (24) |  | 8
Benefits paid |  |  |  | (25) |  | (25)
Contributions by the Group |  |  |  | (28) |  | (25)
Contributions by the plan participants |  |  |  | 0 |  | 0
Closing balance | € 664 |  |  | 664 |  | 670
Other Benefits |  |  |  |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |  |  |  |
Past service cost | (3) | € (2) |  | (2) |  |
Defined benefit obligations |  |  |  |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |  |  |  |
Beginning balance |  |  |  | 1,115 |  | 990
Current service cost |  |  |  | 28 |  | 24
Interest cost / (income) |  |  |  | 19 |  | 26
Past service cost |  |  |  | 2 |  | (1)
Immediate recognition of gains arising over the year |  |  |  | 2 |  | 2
Administration expenses |  |  |  | 0 |  | 0
Remeasurements due to: |  |  |  |  |  |
—actual return less interest on plan assets |  |  |  | 0 |  | 0
—changes in financial assumptions |  |  |  | 67 |  | 126
—changes in demographic assumptions |  |  |  | (10) |  | (4)
—experience losses |  |  |  | (3) |  | (9)
Effects of changes in foreign exchange rates |  |  |  | (44) |  | 19
Benefits paid |  |  |  | (59) |  | (63)
Contributions by the Group |  |  |  | 0 |  | 0
Contributions by the plan participants |  |  |  | 5 |  | 5
Closing balance | 1,122 |  |  | 1,122 |  | 1,115
Defined benefit obligations | Pension Benefits |  |  |  |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |  |  |  |
Beginning balance |  |  |  | 895 |  | 789
Current service cost |  |  |  | 21 |  | 17
Interest cost / (income) |  |  |  | 13 |  | 18
Past service cost |  |  |  | 0 |  | (2)
Immediate recognition of gains arising over the year |  |  |  | 0 |  | 0
Administration expenses |  |  |  | 0 |  | 0
Remeasurements due to: |  |  |  |  |  |
—actual return less interest on plan assets |  |  |  | 0 |  | 0
—changes in financial assumptions |  |  |  | 51 |  | 101
—changes in demographic assumptions |  |  |  | (6) |  | (2)
—experience losses |  |  |  | (4) |  | (3)
Effects of changes in foreign exchange rates |  |  |  | (27) |  | 16
Benefits paid |  |  |  | (41) |  | (43)
Contributions by the Group |  |  |  | 0 |  | 0
Contributions by the plan participants |  |  |  | 4 |  | 4
Closing balance | 906 |  |  | 906 |  | 895
Defined benefit obligations | Other Benefits |  |  |  |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |  |  |  |
Beginning balance |  |  |  | 220 |  | 201
Current service cost |  |  |  | 7 |  | 7
Interest cost / (income) |  |  |  | 6 |  | 8
Past service cost |  |  |  | 2 |  | 1
Immediate recognition of gains arising over the year |  |  |  | 2 |  | 2
Administration expenses |  |  |  | 0 |  | 0
Remeasurements due to: |  |  |  |  |  |
—actual return less interest on plan assets |  |  |  | 0 |  | 0
—changes in financial assumptions |  |  |  | 16 |  | 25
—changes in demographic assumptions |  |  |  | (4) |  | (2)
—experience losses |  |  |  | 1 |  | (6)
Effects of changes in foreign exchange rates |  |  |  | (17) |  | 3
Benefits paid |  |  |  | (18) |  | (20)
Contributions by the Group |  |  |  | 0 |  | 0
Contributions by the plan participants |  |  |  | 1 |  | 1
Closing balance | 216 |  |  | 216 |  | 220
Plan Assets |  |  |  |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |  |  |  |
Beginning balance |  |  |  | (445) |  | (380)
Current service cost |  |  |  | 0 |  | 0
Interest cost / (income) |  |  |  | (8) |  | (10)
Past service cost |  |  |  | 0 |  | 0
Immediate recognition of gains arising over the year |  |  |  | 0 |  | 0
Administration expenses |  |  |  | 2 |  | 2
Remeasurements due to: |  |  |  |  |  |
—actual return less interest on plan assets |  |  |  | (28) |  | (54)
—changes in financial assumptions |  |  |  | 0 |  | 0
—changes in demographic assumptions |  |  |  | 0 |  | 0
—experience losses |  |  |  | 0 |  | 0
Effects of changes in foreign exchange rates |  |  |  | 20 |  | (11)
Benefits paid |  |  |  | 34 |  | 38
Contributions by the Group |  |  |  | (28) |  | (25)
Contributions by the plan participants |  |  |  | (5) |  | (5)
Closing balance | € (458) |  |  | € (458) |  | € (445)

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- Definition The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph c -Clause ii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_c_ii&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph c -Clause iii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_c_iii&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from experience adjustments that result in remeasurements of the net defined benefit liability (asset). Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_c&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The amount of expenses that the entity classifies as being administrative. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 26 -IssueDate 2020-01-01 -Paragraph 35 -Subparagraph b -Clause vi -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=26&code=ifrs-tx-2020-en-r&anchor=para_35_b_vi&doctype=Standard -URIDate 2020-01-01 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 103 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_103&doctype=Standard -URIDate 2020-01-01 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 99 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_99&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The amount of benefits paid or payable for retirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 26 -IssueDate 2020-01-01 -Paragraph 35 -Subparagraph b -Clause v -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=26&code=ifrs-tx-2020-en-r&anchor=para_35_b_v&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan by plan participants. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph f -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_f&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_a&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The amount of employer contributions to retirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 26 -IssueDate 2020-01-01 -Paragraph 35 -Subparagraph b -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=26&code=ifrs-tx-2020-en-r&anchor=para_35_b_i&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_e&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from past service cost. Past service cost is the change in the present value of the defined benefit obligation for employee service in prior periods, resulting from a plan amendment (the introduction or withdrawal of, or changes to, a defined benefit plan) or a curtailment (a significant reduction by the entity in the number of employees covered by a plan). [Refer: Net defined benefit liability (asset); Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph d -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_d&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets, excluding amounts included in interest income or expense. The return on plans assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]; Interest expense (income), net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph c -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_c_i&doctype=Standard -URIDate 2020-01-01 + Details
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