Inventory

12 Months Ended

Dec. 31, 2017

Inventory Disclosure [Abstract]

INVENTORY

INVENTORY

Inventory is valued primarily using the average-cost method. The following table summarizes the Company's inventory balances as of the dates indicated (in millions):

December 31,	2017		2016
Fuel and other raw materials	$	284	$	302
Spare parts and supplies	278		320
Total	$	562	$	622