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Inventory (Tables)
12 Months Ended
Dec. 31, 2017
Inventory Disclosure [Abstract]  
Schedule of Inventory
Inventory consisted of:
 
As of December 31,
 
2017
 
2016
 
(In millions)
Fuel oil
$
90

 
$
142

Coal/Lignite
126

 
219

Natural gas
24

 
28

Spare parts
292

 
332

Total Inventory
$
532


$
721