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Inventory (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Inventory Disclosure Abstract
Coal, fuel oil and other raw materials	$ 363	$ 373
Spare parts and supplies	392	393
Total	$ 755	$ 766

Gross amount of products used directly or indirectly in the manufacturing or production process, which may or may not become part of the final product. May also include items used in the storage, presentation or transportation of physical goods.

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