Inventory (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Inventory Long Term [Abstract]
Long-term emission allowances excluded from inventory	$ 28	$ 33
Public Utilities Inventory [Line Items]
Inventory	1,438	1,226
Fuel for production
Public Utilities Inventory [Line Items]
Inventory	681	542
Material and supplies
Public Utilities Inventory [Line Items]
Inventory	747	676
Emission allowances
Public Utilities Inventory [Line Items]
Inventory	4	8
Other inventory
Public Utilities Inventory [Line Items]
Inventory	$ 6	$ 0

The period end amount for a type of inventory held by the utility in a schedule of inventories.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 43

-Chapter 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Inventory

-URI http://asc.fasb.org/extlink&oid=6516093

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-05.6(a))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_PublicUtilitiesInventory
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant