Net operating costs (excluding items shown separately) - Summary of net operating costs (excluding items shown separately) (Parenthetical) (Detail) £ in Millions, $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Disclosure Of Operating Costs [Line Items]
Impact of (Increase)/decrease in finished goods and work in progress			$ 304.0	$ 143.0
Purchases of the other investors' share of production	$ 229.0		383.0	$ 378.0
Operating lease expense	555.0		$ 541.0
United Kingdoms Financial Conduct Authority (FCA) [Member]
Disclosure Of Operating Costs [Line Items]
Financial penalty	$ 36.4	£ 27.4

The portion of operating lease payments that the lessee is or can be required to make, excluding contingent rent, costs for services and taxes to be paid by, and reimbursed to, the lessor, together with any amounts guaranteed by the lessee or by a party related to the lessee recognised as an expense.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2019-01-01
-Name IAS
-Number 17
-IssueDate 2016-01-13
-Paragraph 35
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=17&date=2016-03-30&anchor=para_35_c&doctype=Standard
-URIDate 2016-03-31

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