Employees - Summary of Movements in Assets (Detail) - EUR (€) € in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of defined benefit plans [Line Items]
Settlements	€ 4	€ (2)	€ 6
Actual return on plan assets (excluding amounts in net finance income/charge)	1,475	1,877	(254)
Assets [member]
Disclosure of defined benefit plans [Line Items]
Beginning balance	21,183	20,742
Employee contributions	18	17
Settlements	(8)
Actual return on plan assets (excluding amounts in net finance income/charge)	1,475	1,877
Interest income	540	664
Employer contributions	1,105	512
Benefit payments	(1,239)	(1,326)
Reclassification of benefits	(1)	(2)
Currency retranslation	(691)	(1,301)
Ending balance	22,382	21,183	20,742
Assets [member] \| United Kingdom [member]
Disclosure of defined benefit plans [Line Items]
Beginning balance	9,963	9,950
Actual return on plan assets (excluding amounts in net finance income/charge)	863	1,412
Interest income	270	329
Employer contributions	778	202
Benefit payments	(457)	(456)
Currency retranslation	(379)	(1,474)
Ending balance	11,038	9,963	9,950
Assets [member] \| Netherlands [member]
Disclosure of defined benefit plans [Line Items]
Beginning balance	5,116	4,873
Employee contributions	1
Actual return on plan assets (excluding amounts in net finance income/charge)	275	281
Interest income	91	120
Employer contributions	43	11
Benefit payments	(169)	(169)
Ending balance	5,357	5,116	4,873
Assets [member] \| Rest of World [member]
Disclosure of defined benefit plans [Line Items]
Beginning balance	6,104	5,919
Employee contributions	17	17
Settlements	(8)
Actual return on plan assets (excluding amounts in net finance income/charge)	337	184
Interest income	179	215
Employer contributions	284	299
Benefit payments	(613)	(701)
Reclassification of benefits	(1)	(2)
Currency retranslation	(312)	173
Ending balance	€ 5,987	€ 6,104	€ 5,919

The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2016-01-13
-Paragraph 141
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_f&doctype=Standard
-URIDate 2016-03-31

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Balance Type:	debit
Period Type:	duration

- 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

+ Details

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- Definition

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.

+ References

No definition available.

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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 2016-01-13
-Paragraph 141
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_e&doctype=Standard
-URIDate 2016-03-31

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan in respect of settlements. [Refer: Gains (losses) arising from settlements, net defined benefit liability (asset); Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2016-01-13
-Paragraph 141
-Subparagraph g
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_g&doctype=Standard
-URIDate 2016-03-31

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets. 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]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2016-01-13
-Paragraph 141
-Subparagraph c
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_c_i&doctype=Standard
-URIDate 2016-03-31

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Balance Type:	debit
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- Definition

The present value of the defined benefit obligation, less the fair value of the plan assets (if any). [Refer: Plan assets [member]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2016-01-13
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_57_a&doctype=Standard
-URIDate 2016-03-31

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- Definition

Benefit Payments.

+ References

No definition available.

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- Definition

Income as return on plan assets.

+ References

No definition available.

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- Definition

Reclassification of benefits.

+ References

No definition available.

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