Interest in associates and joint ventures - Joint ventures commentary (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Interests In Other Entities [Abstract]
Carrying amount of interests in joint ventures	$ 167	$ 155

The amount of investments in joint ventures. [Refer: Joint ventures [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 27
-IssueDate 2017-01-01
-Paragraph 10
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=27&date=2017-03-01&anchor=para_10&doctype=Standard
-URIDate 2017-03-09

Name:	ifrs-full_InvestmentsInJointVentures
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant