Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount before deducting deferred taxes primariliy related to appreciation of investment value	$ 292		$ 240
Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	214		209
Total assets	2,523		2,241
Total liabilities	59		163
Other investments	51
Fair Value Measurements, Level 1 [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	26		21
U.S. equity securities	1,009		940
Non-U.S. equity securities	435		379
U.S. government and agency securities	782		681
Total assets	2,252	[1]	2,021	[1]
Fair Value Measurements, Level 1 [Member] | Price Risk Management Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	1		1
Electricity	1		1
Electricity	53		155
Natural Gas	6		8
Fair Value Measurements, Level 1 [Member] | Long-Term Disability Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	5		10
Total assets	5		10
Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	580		708
Total liabilities	105		153
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	10		9
U.S. government and agency securities	148		139
Municipal securities	26		59
Other fixed-income securities	128		173
Total assets	312	[1]	380	[1]
Fair Value Measurements, Level 2 [Member] | Price Risk Management Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	31		65
Electricity	27		60
Natural Gas	4		5
Electricity	100		144
Natural Gas	5		9
Fair Value Measurements, Level 2 [Member] | Rabbi Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	100		102
Fixed-income securities	30		30
Life insurance contracts	70		72
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	11		14
Non-U.S. equity securities	10		11
Total assets	137		161
Fixed-income securities	116		136
Fair Value Measurements, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	65		81
Total liabilities	147		160
Fair Value Measurements, Level 3 [Member] | Price Risk Management Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	65		81
Electricity	65		80
Natural Gas	0		1
Electricity	147		160
Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	2	[2]	0	[2]
Total liabilities	(146)	[2]	(165)	[2]
Netting [Member] | Price Risk Management Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	2	[2]	0	[2]
Electricity	2	[2]	6	[2]
Natural Gas	0	[2]	(6)	[2]
Electricity	(140)	[2]	(156)	[2]
Natural Gas	(6)	[2]	(9)	[2]
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	214		209
Total assets	3,170		3,030
Total liabilities	165		311
Other investments	51
Fair Value [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	26		21
U.S. equity securities	1,019		949
Non-U.S. equity securities	435		379
U.S. government and agency securities	930		820
Municipal securities	26		59
Other fixed-income securities	128		173
Total assets	2,564	[1]	2,401	[1]
Fair Value [Member] | Price Risk Management Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	99		147
Electricity	95		147
Natural Gas	4		0
Electricity	160		303
Natural Gas	5		8
Fair Value [Member] | Rabbi Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	100		102
Fixed-income securities	30		30
Life insurance contracts	70		72
Fair Value [Member] | Long-Term Disability Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	5		10
U.S. equity securities	11		14
Non-U.S. equity securities	10		11
Total assets	142		171
Fixed-income securities	$ 116		$ 136

[1] Represents amounts before deducting $292 million and $240 million at September 30, 2013 and December 31, 2012, respectively, of deferred taxes primarily related to appreciation of investment value.

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