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Inventory (Tables)
3 Months Ended
Mar. 31, 2019
Inventory Disclosure [Abstract]  
Inventory Balance By Type The following table summarizes the Company’s inventory balances as of the periods indicated (in millions):
 
March 31, 2019
 
December 31, 2018
Fuel and other raw materials
$
310

 
$
300

Spare parts and supplies
269

 
277

Total
$
579

 
$
577