Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount primarily related to deferred taxes on appreciation of investment value	$ 188		$ 248
Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	206		206
Total assets	2,127		2,228
Total liabilities	442		451
Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	680		700
Total liabilities	302		295
Fair Value Measurements, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	69		67
Total liabilities	143		166
Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments		[1]
Nuclear decomissioning trusts		[1]
Total assets	5	[1]	27	[1]
Total liabilities	(473)	[1]	(489)	[1]
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	206		206
Total assets	2,881		3,022
Total liabilities	414		423
Nuclear Decommissioning Trusts [Member] | Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	841		925
Non-U.S. equity securities	323		358
U.S. government and agency securities	744		734
Total assets	1,908	[2]	2,017	[2]
Nuclear Decommissioning Trusts [Member] | Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	8		9
U.S. government and agency securities	156		138
Municipal securities	58		68
Other fixed-income securities	99		150
Total assets	321	[2]	365	[2]
Nuclear Decommissioning Trusts [Member] | Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities		[1]
Non-U.S. equity securities		[1]
U.S. government and agency securities		[1]
Municipal securities		[1]
Other fixed-income securities		[1]
Total assets		[1],[2]
Nuclear Decommissioning Trusts [Member] | Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	849		934
Non-U.S. equity securities	323		358
U.S. government and agency securities	900		872
Municipal securities	58		68
Other fixed-income securities	99		150
Total assets	2,229	[2]	2,382	[2]
Price Risk Management Instruments [Member] | Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Electricity, Liabilities	411		436
Gas, Liabilities	31		15
Price Risk Management Instruments [Member] | Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	98		71
Electricity, Assets	92		68
Gas, Assets	6		3
Electricity, Liabilities	289		284
Gas, Liabilities	13		11
Price Risk Management Instruments [Member] | Fair Value Measurements, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	69		67
Electricity, Assets	69		67
Electricity, Liabilities	143		166
Price Risk Management Instruments [Member] | Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	5	[1]	27	[1]
Electricity, Assets	8	[1]	27	[1]
Gas, Assets	(3)	[1]
Electricity, Liabilities	(441)	[1]	(471)	[1]
Gas, Liabilities	(32)	[1]	(18)	[1]
Price Risk Management Instruments [Member] | Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	172		165
Electricity, Assets	169		162
Gas, Assets	3		3
Electricity, Liabilities	402		415
Gas, Liabilities	12		8
Rabbi Trusts [Member] | Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other fixed-income securities	25		26
Life insurance contracts	67		68
Total assets	92		94
Rabbi Trusts [Member] | Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other fixed-income securities		[1]
Life insurance contracts		[1]
Total assets		[1]
Rabbi Trusts [Member] | Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other fixed-income securities	25		26
Life insurance contracts	67		68
Total assets	92		94
Long-Term Disability Trust [Member] | Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	13		5
Total assets	13		5
Long-Term Disability Trust [Member] | Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	15		13
Non-U.S. equity securities	9		15
Fixed-income securities	145		142
Total assets	169		170
Long-Term Disability Trust [Member] | Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities		[1]
Non-U.S. equity securities		[1]
Fixed-income securities		[1]
Total assets		[1]
Long-Term Disability Trust [Member] | Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	28		18
Non-U.S. equity securities	9		15
Fixed-income securities	145		142
Total assets	$ 182		$ 175

[1] Includes the effect of the contractual ability to settle contracts under master netting agreements and margin cash collateral.

[2] Excludes $248 million and $188 million at March 31, 2012 and December 31, 2011, respectively, primarily related to deferred taxes on appreciation of investment value.