Trade and Other Liabilities - Additional Information (Detail) - IFRS 15 [member] £ in Millions	12 Months Ended
Dec. 31, 2018 GBP (£)
Trade and other payables [line items]
Sales return liability	£ 173
Increase in deferred income	£ 28

Adjustments for increase (decrease) in deferred income to reconcile profit (loss) to net cash flow from (used in) operating activities. [Refer: Deferred income; Profit (loss)]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 7
-IssueDate 2018-01-01
-Paragraph 20
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2018-03-01&anchor=para_20&doctype=Standard
-URIDate 2018-03-16

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration