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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
	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)

X

- Definition

Immediate recognition actuarial gains (losses) arising from change in assumption recognized in profit (loss).

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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)]

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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)]

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The amount of expenses that the entity classifies as being administrative.

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The amount of benefits paid or payable for retirement benefit plans.

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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]]

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The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

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The amount of employer contributions to retirement benefit plans.

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The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

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The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

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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]]

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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)]

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