Inventory (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Inventory Net Abstract
Coal, fuel oil and other raw materials	$ 351	$ 276
Spare parts and supplies	303	286
Total	$ 654	$ 562

Carrying amount as of the balance sheet date of unprocessed items to be consumed in the manufacturing or production process. Also includes purchased parts that will be used as components of a finished product.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 6
-Subparagraph a
-Article 5

Name:	us-gaap_InventoryRawMaterials
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant