Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 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	$ 306		$ 313
Long-Term Disability Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	4
Total assets	4
Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	53		226
Total assets	2,408		2,622
Total liabilities	12		20
Other Investments	36		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	16		38
U.S. equity securities	1,118		1,046
Non-U.S. equity securities	432		457
U.S. government and agency securities	745		760
Total assets	2,311	[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	4		2
Electricity	1		2
Electricity	9		19
Natural Gas	3		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			9
Total assets			9
Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	667		643
Total liabilities	17		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	164		156
Municipal securities	54		25
Other fixed-income securities	163		162
Total assets	394	[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	33		32
Electricity	29		27
Natural Gas	4		5
Electricity	17		72
Natural Gas	0		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	112		109
Fixed-income securities	41		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	9		14
Non-U.S. equity securities	12		12
Total assets	128		148
Fixed-income securities	107		122
Fair Value Measurements, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	95		107
Total liabilities	115		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	95		107
Electricity	95		107
Electricity	115		137
Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	5	[3]	2	[3]
Total liabilities	(16)	[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	(16)	[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	53		226
Total assets	3,175		3,374
Total liabilities	128		147
Other Investments	36		84
Fair Value [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	16		38
U.S. equity securities	1,130		1,057
Non-U.S. equity securities	433		457
U.S. government and agency securities	909		916
Municipal securities	54		25
Other fixed-income securities	163		162
Total assets	2,705	[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	137		143
Electricity	130		139
Natural Gas	7		4
Electricity	125		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	112		109
Fixed-income securities	41		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	4		9
U.S. equity securities	9		14
Non-U.S. equity securities	12		12
Total assets	132		157
Fixed-income securities	$ 107		$ 122

[1] Represents amount before deducting $306 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.