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Inventory
12 Months Ended
Dec. 31, 2015
Inventory Disclosure [Abstract]  
INVENTORY
INVENTORY
Inventory is valued primarily using the average-cost method. The following table summarizes the Company's inventory balances (in millions) as of the dates indicated:
December 31,
 
2015
 
2014
Fuel and other raw materials
 
$
343

 
$
357

Spare parts and supplies
 
332

 
345

Total
 
$
675

 
$
702