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Inventory (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Inventory Disclosure Abstract
Coal, fuel oil and other raw materials	$ 432	$ 444
Spare parts and supplies	367	341
Total	$ 799	$ 785

Carrying amount as of the balance sheet date 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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