Risk Management Activities - Schedule of Combined Maximum Credit Risk Exposure (Detail) - Credit risk [member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Environmental trust funds [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	$ 55.5	$ 44.5
Trade and other receivable [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	66.5	68.5
Cash and cash equivalent [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	$ 479.0	$ 526.7

The amount that best represents the maximum exposure to credit risk without taking into account any collateral held or other credit enhancements (for example, netting agreements that do not qualify for offset in accordance with IAS 32). [Refer: Credit risk [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 35K
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_35K_a&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 36
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_36_a&doctype=Standard&book=B
-URIDate 2017-03-09

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