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Inventory (Tables)
 12 Months Ended
Dec. 31, 2020
Disclosure of inventories [Abstract]  
Disclosure Of The Components Of Inventories
The components of inventory are as follows:
As at Dec. 3120202019
Parts and materials107 108 
Coal83 130 
Deferred stripping costs8 6 
Natural gas2 3 
Purchased emission credits(1)
38 4 
Total238 251 
(1) Purchased emissions credits increased due to trading and compliance credits purchased, including those for Alberta compliance under the Technology Innovation and Emissions Reduction program.
Disclosure Of The Change In Inventory
The change in inventory is as follows:
Balance, Dec. 31, 2018242 
Net addition12 
Change in foreign exchange rates(3)
Balance, Dec. 31, 2019251 
Net addition26 
Writedowns(37)
Change in foreign exchange rates(2)
Balance, Dec. 31, 2020238