Debt and Credit Agreements (Details) (USD $) In Millions, unless otherwise specified						1 Months Ended				1 Months Ended											3 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Southern California Edison	Dec. 31, 2011 Southern California Edison	Sep. 30, 2012 Edison Mission Energy Short-term Debt [Member]	Mar. 30, 2012 Broken Bow and Crofton Bluffs Wind Projects	Sep. 30, 2012 Big Sky	Oct. 31, 2009 Big Sky MW	Mar. 31, 2012 First and refunding mortgage bonds Southern California Edison	Dec. 31, 2011 Term Loan Tapestry Wind, LLC	Oct. 31, 2009 Vendor financing loan due 2014 Big Sky	Sep. 30, 2012 Letters of Credit	Sep. 30, 2012 Letters of Credit Southern California Edison	Sep. 30, 2012 Letters of Credit Edison Mission Energy	Sep. 30, 2012 Letters of Credit Subsidiaries	Sep. 30, 2012 Letters of Credit Walnut Creek credit facility	Sep. 30, 2012 Revolving credit facility Southern California Edison	Feb. 29, 2012 Revolving credit facility Edison Mission Energy	Sep. 30, 2012 Revolving credit facility Midwest Generation LLC.	Mar. 31, 2012 Revolving credit facility terminating June 2014 Edison Mission Energy	Sep. 30, 2012 Revolving credit facility maturing in May 2017	Dec. 31, 2011 Revolving credit facility maturing in May 2017
Debt Instrument:
Construction loans						$ 79
Long-term debt - Gross									400	214
Vendor financing max borrowing amount											206
Term of amortizing loan (in years)						15 years				10 years											5 years
Letter of credit facilities						13
Working capital facilities						6
Floating interest rate received						LIBOR
Variable rate, spread (as a percent)						2.875%
Variable rate, increase rate after third, sixth, ninth and twelfth years (as a percent)						0.125%
Construction loan, amount outstanding					21
Interest rate on debt (as a percent)									4.05%
Cash proceeds received from issuance of debt										97
Outstanding short-term debt	429	429	380	419																	28	10
Short-term debt, weighted average interest rate (as a percent)			0.43%	0.44%																	1.52%	0.66%
Credit facility																	2,750	564	500	100	1,250
Outstanding credit facility													(196)
Letter of credit expiration date																	5 years
Standby letters of credit												154		51	103	30
Standby letters of credit expiring in 2012															2
Standby letters of credit expiring in 2013														48	49
Standby letters of credit expiring in 2014														3
Standby letters of credit expiring in 2017															21
Standby letters of credit expiring in 2018															18
Standby letters of credit expiring in 2021															13
Outstanding letters of credit					6
Power generating capacity for majority interest (in megawatts)								240
Debt instrument, final maturity period											5 years
Investments in unconsolidated affiliates	$ 544	$ 525					$ 130