Consolidated Statements of Changes in Equity (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2011
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, at the beginning of the period			$ 11,086		$ 11,494
Net income (loss)	220	441	435		845
Other comprehensive loss	(11)	(27)	(29)		(48)
Contributions from noncontrolling interests			238	[1]
Transfer of assets to Capistrano Wind Partners			(21)	[2]
Common stock dividends declared ($0.975 and $0.96 per share, 2012 and 2011 respectively)			(318)		(313)
Dividends, distributions to noncontrolling interests and other			(80)		(45)
Stock-based compensation and other			(45)		(14)
Noncash stock-based compensation and other			25		18
Purchase of noncontrolling interest					(14)	[3]
Issuance of preference stock			804		123
Redemption of preference stock			(75)
Balance, at the end of the period	12,020	12,046	12,020		12,046
Dividends declared per common share (in dollars per share)	$ 0.325	$ 0.320	$ 0.975		$ 0.960
EMG | Pinnacle Wind Force, LLC, and Broken Bow I, LLC and all assets of the Crofton Bluffs project
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Ownership interest (as a percentage)					100.00%
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, at the beginning of the period			2,360		2,331
Net income (loss)			0		0
Other comprehensive loss			0		0
Contributions from noncontrolling interests			0	[1]
Transfer of assets to Capistrano Wind Partners			(21)	[2]
Common stock dividends declared ($0.975 and $0.96 per share, 2012 and 2011 respectively)			0		0
Dividends, distributions to noncontrolling interests and other			0		0
Stock-based compensation and other			19		7
Noncash stock-based compensation and other			27		22
Purchase of noncontrolling interest					(14)	[3]
Issuance of preference stock			0		0
Redemption of preference stock			0
Balance, at the end of the period	2,385	2,346	2,385		2,346
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, at the beginning of the period			(139)		(76)
Net income (loss)			0		0
Other comprehensive loss			(29)		(48)
Contributions from noncontrolling interests			0	[1]
Transfer of assets to Capistrano Wind Partners			0	[2]
Common stock dividends declared ($0.975 and $0.96 per share, 2012 and 2011 respectively)			0		0
Dividends, distributions to noncontrolling interests and other			0		0
Stock-based compensation and other			0		0
Noncash stock-based compensation and other			0		0
Purchase of noncontrolling interest					0	[3]
Issuance of preference stock			0		0
Redemption of preference stock			0
Balance, at the end of the period	(168)	(124)	(168)		(124)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, at the beginning of the period			7,834		8,328
Net income (loss)			357		802
Other comprehensive loss			0		0
Contributions from noncontrolling interests			0	[1]
Transfer of assets to Capistrano Wind Partners			0	[2]
Common stock dividends declared ($0.975 and $0.96 per share, 2012 and 2011 respectively)			(318)		(313)
Dividends, distributions to noncontrolling interests and other			0		0
Stock-based compensation and other			(64)		(21)
Noncash stock-based compensation and other			(2)		(3)
Purchase of noncontrolling interest					0	[3]
Issuance of preference stock			0		0
Redemption of preference stock			(1)
Balance, at the end of the period	7,806	8,793	7,806		8,793
Subtotal
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, at the beginning of the period			10,055		10,583
Net income (loss)			357		802
Other comprehensive loss			(29)		(48)
Contributions from noncontrolling interests			0	[1]
Transfer of assets to Capistrano Wind Partners			(21)	[2]
Common stock dividends declared ($0.975 and $0.96 per share, 2012 and 2011 respectively)			(318)		(313)
Dividends, distributions to noncontrolling interests and other			0		0
Stock-based compensation and other			(45)		(14)
Noncash stock-based compensation and other			25		19
Purchase of noncontrolling interest					(14)	[3]
Issuance of preference stock			0		0
Redemption of preference stock			(1)
Balance, at the end of the period	10,023	11,015	10,023		11,015
Other
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, at the beginning of the period			2		4
Net income (loss)			12		(1)
Other comprehensive loss			0		0
Contributions from noncontrolling interests			238	[1]
Transfer of assets to Capistrano Wind Partners			0	[2]
Common stock dividends declared ($0.975 and $0.96 per share, 2012 and 2011 respectively)			0		0
Dividends, distributions to noncontrolling interests and other			(14)		(1)
Stock-based compensation and other			0		0
Noncash stock-based compensation and other			0		0
Purchase of noncontrolling interest					0	[3]
Issuance of preference stock			0		0
Redemption of preference stock			0
Balance, at the end of the period	238	2	238		2
Preferred Stock and Preference Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, at the beginning of the period			1,029		907
Net income (loss)			66		44
Other comprehensive loss			0		0
Contributions from noncontrolling interests			0	[1]
Transfer of assets to Capistrano Wind Partners			0	[2]
Common stock dividends declared ($0.975 and $0.96 per share, 2012 and 2011 respectively)			0		0
Dividends, distributions to noncontrolling interests and other			(66)		(44)
Stock-based compensation and other			0		0
Noncash stock-based compensation and other			0		(1)
Purchase of noncontrolling interest					0	[3]
Issuance of preference stock			804		123
Redemption of preference stock			(74)
Balance, at the end of the period	$ 1,759	$ 1,029	$ 1,759		$ 1,029

[1] Funds contributed by third-party investors related to the Capistrano Wind equity capital raise are reported in noncontrolling interest. For further information, see Note 3.

[2] Common stock was reduced by $21 million during the nine months ended September 30, 2012 due to a new tax basis in the assets transferred to Capistrano Wind Partners. For further information, see Note 3.

[3] During the nine months ended September 30, 2011, EMG purchased the remaining interests in Pinnacle Wind Force, LLC, and Broken Bow I, LLC and all assets of the Crofton Bluffs project. All three projects are now 100% owned by EMG. The purchases of the noncontrolling interests were accounted for as equity transactions between controlling and noncontrolling interest holders.