Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended										0 Months Ended	3 Months Ended	9 Months Ended				0 Months Ended	3 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012	Sep. 30, 2013 Broken Bow I Project MW	Feb. 13, 2012 Cedro Hill Project MW	Feb. 13, 2012 Mountain Wind Power I Project MW	Feb. 13, 2012 Mountain Wind Power II Project MW	Feb. 13, 2012 Cedro Hill Wind LLC, Mountain Wind Power I and Mountain Wind Power Il	Feb. 13, 2012 Capistrano Wind Partners	Dec. 31, 2012 Capistrano Wind Partners	Sep. 30, 2013 Capistrano Wind Partners	Dec. 31, 2012 Crofton Bluffs MW	Sep. 30, 2013 Variable Interest Entity, Primary Beneficiary projects MW	Dec. 31, 2012 Variable Interest Entity, Primary Beneficiary MW projects	Feb. 13, 2012 Mission Energy Holding Company Capistrano Wind Partners	Sep. 30, 2013 Third Party Investors Capistrano Wind Partners
Details of projects or entities
Number of projects consolidated (in projects)															16	15
Total generating capacity (in megawatts)															958	878
Summarized balance sheet information of VIEs
Current assets	$ 1,525				$ 1,323										$ 84	$ 74
Net property, plant and equipment	3,934				4,516										1,205	1,117
Other long-term assets															108	90
Total Assets	7,054				7,520										1,397	1,281
Current liabilities	238				820										39	50
Long-term debt net of current portion	1,433				749										227	186
Deferred revenues	513				533										153	156
Long-term derivative liabilities	69				118										14	23
Other long-term liabilities	510				528										47	40
Total Liabilities	6,795				6,788										480	455
Noncontrolling interests	392				288										392	288
Assets serving as collateral for the debt obligations	623				497
Sale of ownership (in megawatts)						80	150	61	80					40
Variable interest entities, sales price						112				346				58
Class A equity ownership percentage (as a percent)											100.00%
Preferred equity interests to receive percent of cash available for distribution (as a percent)											100.00%
Cash available for distribution includes percent of tax benefits to be realized (as a percent)											90.00%
Contributions from noncontrolling interests	$ 94	[1]	$ 242	[1]							$ 238	$ 46	$ 94				$ 4	$ 378

[1] Funds contributed by third-party investors to Capistrano Wind Partners. For further information, see Note 3—Variable Interest Entities—Projects or Entities that are Consolidated—Capistrano Wind Partners.