Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount primarily related to deferred taxes on appreciation of investment value	$ 337		$ 313
Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	46		226
Total assets	2,449		2,622
Total liabilities	12		20
Other Investments	93		84
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	31		38
U.S. equity securities	1,087		1,046
Non-U.S. equity securities	439		457
U.S. government and agency securities	741		760
Total assets	2,298	[1]	2,301	[2]
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	6		2
Electricity	6		2
Electricity	12		19
Natural Gas	0		1
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	6		9
Total assets	6		9
Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	691		643
Total liabilities	52		75
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	12		11
Non-U.S. equity securities	1		0
U.S. government and agency securities	166		156
Municipal securities	40		25
Other fixed-income securities	171		162
Total assets	390	[1]	354	[2]
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	47		32
Electricity	38		27
Natural Gas	9		5
Electricity	49		72
Natural Gas	3		3
Fair Value Measurements, Level 2 [Member] | Rabbi Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	111		109
Fixed-income securities	40		39
Life insurance contracts	71		70
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	12		14
Non-U.S. equity securities	11		12
Total assets	143		148
Fixed-income securities	120		122
Fair Value Measurements, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	104		107
Total liabilities	126		137
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	104		107
Electricity	104		107
Electricity	126		137
Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	(5)	[3]	2	[3]
Total liabilities	(49)	[3]	(85)	[3]
Netting [Member] | Price Risk Management Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	(5)	[3]	2	[3]
Electricity	(5)	[3]	3	[3]
Natural Gas	0	[3]	(1)	[3]
Electricity	(49)	[3]	(84)	[3]
Natural Gas	0	[3]	(1)	[3]
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	46		226
Total assets	3,239		3,374
Total liabilities	141		147
Other Investments	93		84
Fair Value [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	31		38
U.S. equity securities	1,099		1,057
Non-U.S. equity securities	440		457
U.S. government and agency securities	907		916
Municipal securities	40		25
Other fixed-income securities	171		162
Total assets	2,688	[1]	2,655	[2]
Fair Value [Member] | Price Risk Management Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	152		143
Electricity	143		139
Natural Gas	9		4
Electricity	138		144
Natural Gas	3		3
Fair Value [Member] | Rabbi Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	111		109
Fixed-income securities	40		39
Life insurance contracts	71		70
Fair Value [Member] | Long-Term Disability Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	6		9
U.S. equity securities	12		14
Non-U.S. equity securities	11		12
Total assets	149		157
Fixed-income securities	$ 120		$ 122

[1] Represents amount before deducting $337 million, primarily related to deferred taxes on appreciation of investment value.

[2] Represents amount before deducting $313 million, primarily related to deferred taxes on appreciation of investment value.

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