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Inventory (Tables)
12 Months Ended
Dec. 31, 2018
Inventory Disclosure [Abstract]  
Inventory Balance By Type The following table summarizes the Company's inventory balances as of the dates indicated (in millions):
December 31,
 
2018
 
2017
Fuel and other raw materials
 
$
300

 
$
284

Spare parts and supplies
 
277

 
278

Total
 
$
577

 
$
562