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Inventory (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of inventories [Abstract]  
Disclosure Of The Components Of Inventories
The components of inventory are as follows:
As at Dec. 31
2017

2016

Parts and materials
118

110

Coal
58

65

Deferred stripping costs
11

12

Natural gas
9

17

Purchased emission credits
23

9

Total
219

213
Disclosure Of The Change In Inventory
The change in inventory is as follows:
Balance, Dec. 31, 2015
219

Net use
(12
)
Writedowns
(9
)
Reversal of writedowns
13

Change in foreign exchange rates
2

Balance, Dec. 31, 2016
213

Net addition
11

Change in foreign exchange rates
(5
)
Balance, Dec. 31, 2017
219