Loans and other borrowings - Summary of Loans and Other Borrowings (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2017 GBP (£)
GBP400m 3.875% bonds 2022 [member]
Disclosure of detailed information about borrowings [line items]
Notional amount	£ 400,000,000
Bond interest rate	3.875%
Bond maturity period	28 November 2022
GBP300m 3.75% bonds 2025 [member]
Disclosure of detailed information about borrowings [line items]
Notional amount	£ 300,000,000
Bond interest rate	3.75%
Bond maturity period	14 August 2025
GBP350m 2.125% bonds 2026 [member]
Disclosure of detailed information about borrowings [line items]
Notional amount	£ 350,000,000
Bond interest rate	2.125%
Bond maturity period	24 August 2026

The interest rate on borrowings. [Refer: Borrowings]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-URIDate 2017-03-09

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- Definition

The maturity of borrowings. [Refer: Borrowings]

+ References

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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 nominal or face amount of a financial instrument, used to calculate payments made on that instrument.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
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-IssueDate 2017-01-01
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-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
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