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Inventory (Tables)
6 Months Ended
Jun. 30, 2017
Inventory Disclosure [Abstract]  
Inventory Balance By Type
The following table summarizes the Company’s inventory balances as of the periods indicated (in millions):
 
June 30, 2017
 
December 31, 2016
Fuel and other raw materials
$
330

 
$
302

Spare parts and supplies
303

 
328

Total
$
633

 
$
630