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Inventory (Notes)
 12 Months Ended
Dec. 31, 2019
Disclosure of inventories [Abstract]  
Inventory Inventory
Inventory held in the normal course of business, which includes coal, emission credits, parts and materials, and natural gas, is valued at the lower of cost and net realizable value. Inventory held for trading, which includes natural gas and emission credits and allowances, is valued at fair value less costs to sell.
The components of inventory are as follows:
As at Dec. 3120192018
Parts and materials108  113  
Coal130  108  
Deferred stripping costs6  7  
Natural gas3  4  
Purchased emission credits4  10  
Total251  242  

The change in inventory is as follows:
Balance, Dec. 31, 2017219  
Net addition20  
Change in foreign exchange rates3  
Balance, Dec. 31, 2018242  
Net addition12  
Change in foreign exchange rates(3) 
Balance, Dec. 31, 2019251  

No inventory is pledged as security for liabilities.