Financial instruments - Hedging Activities (Details) - Foreign currency translation risk	Dec. 31, 2017 $ / £ € / £ $ / £	Dec. 31, 2016 $ / £ € / £ $ / £
Sales hedges \| U.S. dollar
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Contract totals	17.1	27.6
Sales hedges \| Euros
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Contract totals \| € / £	27.5	39.4
Sales hedges \| Australian dollar
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Contract totals	2.8	2.9
Exchange rates	1.7667	1.7237
Sales hedges \| Minimum \| U.S. dollar
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Exchange rates	1.2433	1.2310
Sales hedges \| Minimum \| Euros
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Exchange rates \| € / £	1.0949	1.0951
Sales hedges \| Maximum \| U.S. dollar
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Exchange rates	1.3444	1.5638
Sales hedges \| Maximum \| Euros
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Exchange rates \| € / £	1.1803	1.4200
Purchase hedges \| U.S. dollar
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Contract totals	12.5	25.7
Purchase hedges \| Euros
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Contract totals \| € / £	0.1	2.5
Exchange rates \| € / £	1.1804
Purchase hedges \| Australian dollar
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Contract totals		1.7
Purchase hedges \| Minimum \| U.S. dollar
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Exchange rates	1.2414	1.2311
Purchase hedges \| Minimum \| Euros
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Exchange rates \| € / £		1.1121
Purchase hedges \| Maximum \| U.S. dollar
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Exchange rates	1.3389	1.5618
Purchase hedges \| Maximum \| Euros
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items]
Exchange rates \| € / £		1.1804

The average price of a hedging instrument. [Refer: Hedging instruments [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 23B
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_23B_b&doctype=Standard
-URIDate 2017-03-09

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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 amount of a hedging instrument. [Refer: Hedging instruments [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 23B
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_23B_a&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 24A
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_24A_d&doctype=Standard
-URIDate 2017-03-09

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