Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 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	$ 266		$ 240
Other investments	56
Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	222		209
Total assets	2,420		2,241
Total liabilities	91		163
Other investments	56
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	32		21
U.S. equity securities	956		940
Non-U.S. equity securities	391		379
U.S. government and agency securities	759		681
Total assets	2,138	[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	0		1
Natural Gas	1
Electricity	85		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	3		10
Total assets	3		10
Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	619		708
Total liabilities	117		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	135		139
Municipal securities	31		59
Other fixed-income securities	166		173
Total assets	342	[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	29		65
Electricity	26		60
Natural Gas	3		5
Electricity	112		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	29		30
Life insurance contracts	71		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	13		14
Non-U.S. equity securities	12		11
Total assets	148		161
Fixed-income securities	123		136
Fair Value Measurements, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	72		81
Total liabilities	148		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	72		81
Electricity	72		80
Natural Gas	0		1
Electricity	148		160
Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	12	[2]	0	[2]
Total liabilities	(189)	[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	12	[2]	0	[2]
Electricity	15	[2]	6	[2]
Natural Gas	(3)	[2]	(6)	[2]
Electricity	(183)	[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	222		209
Total assets	3,123		3,030
Total liabilities	167		311
Fair Value [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	32		21
U.S. equity securities	966		949
Non-U.S. equity securities	391		379
U.S. government and agency securities	894		820
Municipal securities	31		59
Other fixed-income securities	166		173
Total assets	2,480	[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	114		147
Electricity	113		147
Natural Gas	1		0
Electricity	162		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	29		30
Life insurance contracts	71		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	3		10
U.S. equity securities	13		14
Non-U.S. equity securities	12		11
Total assets	151		171
Fixed-income securities	$ 123		$ 136

[1] Excludes $266 million and $240 million at June 30, 2013 and December 31, 2012, 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.