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Inventory
12 Months Ended
Dec. 31, 2019
Inventory Disclosure [Abstract]  
INVENTORY INVENTORY
Inventory is valued primarily using the average-cost method. The following table summarizes the Company's inventory balances as of the dates indicated (in millions):
December 31,
 
2019
 
2018
Fuel and other raw materials
 
$
230

 
$
300

Spare parts and supplies
 
257

 
277

Total
 
$
487

 
$
577