Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount primarily related to deferred taxes on appreciation of investment value	$ 210		$ 188
Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	245		206
Total assets	2,179		2,127
Total liabilities	326		442
Fair Value Measurements, Level 1 [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	891		841
Non-U.S. equity securities	330		323
U.S. government and agency securities	709		744
Total assets	1,930	[1]	1,908	[1]
Fair Value Measurements, Level 1 [Member] | Price Risk Management Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Electricity	312		411
Gas	14		31
Fair Value Measurements, Level 1 [Member] | Long-Term Disability Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	4		13
Total assets	4		13
Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	725		680
Total liabilities	230		302
Fair Value Measurements, Level 2 [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	8		8
U.S. government and agency securities	142		156
Municipal securities	57		58
Other fixed-income securities	179		99
Total assets	386	[1]	321	[1]
Fair Value Measurements, Level 2 [Member] | Price Risk Management Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	81		98
Electricity	75		92
Gas	6		6
Electricity	220		289
Gas	10		13
Fair Value Measurements, Level 2 [Member] | Rabbi Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	96		92
Fixed-income securities	27		25
Life insurance contracts	69		67
Fair Value Measurements, Level 2 [Member] | Long-Term Disability Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	13		15
Non-U.S. equity securities	13		9
Total assets	162		169
Fixed-income securities	136		145
Fair Value Measurements, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	72		69
Total liabilities	152		143
Fair Value Measurements, Level 3 [Member] | Price Risk Management Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	72		69
Electricity	70		69
Gas	2
Electricity	152		143
Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	14	[2]	5	[2]
Total liabilities	(331)	[2]	(473)	[2]
Netting [Member] | Price Risk Management Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	14	[2]	5	[2]
Electricity	20	[2]	8	[2]
Gas	(6)	[2]	(3)	[2]
Electricity	(315)	[2]	(441)	[2]
Gas	(16)	[2]	(32)	[2]
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	245		206
Total assets	2,990		2,881
Total liabilities	377		414
Fair Value [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	899		849
Non-U.S. equity securities	330		323
U.S. government and agency securities	851		900
Municipal securities	57		58
Other fixed-income securities	179		99
Total assets	2,316	[1]	2,229	[1]
Fair Value [Member] | Price Risk Management Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	167		172
Electricity	165		169
Gas	2		3
Electricity	369		402
Gas	8		12
Fair Value [Member] | Rabbi Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	96		92
Fixed-income securities	27		25
Life insurance contracts	69		67
Fair Value [Member] | Long-Term Disability Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. equity securities	17		28
Non-U.S. equity securities	13		9
Total assets	166		182
Fixed-income securities	$ 136		$ 145

[1] Excludes $210 million and $188 million at June 30, 2012 and December 31, 2011, respectively, primarily related to deferred taxes on appreciation of investment value.

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