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Intangible Exploration and Evaluation Assets (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of detailed information about intangible assets [abstract]  
Schedule of intangible exploration and evaluation assets
(000s)
 
Balance at December 31, 2015
$
122,020

Additions
19,425

Transfer to petroleum properties
(2,150
)
Impairment loss
(33,426
)
Balance at December 31, 2016
105,869

Additions
16,905

Transfer to petroleum and natural gas assets
(2,271
)
Impairment loss
(79,025
)
Balance at December 31, 2017
$
41,478