Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 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	$ 322		$ 313
Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	63		226
Total assets	2,480		2,622
Total liabilities	6		20
Other Investments	71		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	19		38
U.S. equity securities	1,128		1,046
Non-U.S. equity securities	457		457
U.S. government and agency securities	732		760
Total assets	2,336	[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	5		2
Electricity	5		2
Electricity	6		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	5		9
Total assets	5		9
Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	703		643
Total liabilities	33		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	2		0
U.S. government and agency securities	173		156
Municipal securities	55		25
Other fixed-income securities	172		162
Total assets	414	[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	39		32
Electricity	31		27
Natural Gas	8		5
Electricity	29		72
Natural Gas	4		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	113		109
Fixed-income securities	41		39
Life insurance contracts	72		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	137		148
Fixed-income securities	114		122
Fair Value Measurements, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	105		107
Total liabilities	116		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	105		107
Electricity	105		107
Electricity	116		137
Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0	[3]	2	[3]
Total liabilities	(26)	[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	0	[3]	2	[3]
Electricity	1	[3]	3	[3]
Natural Gas	(1)	[3]	(1)	[3]
Electricity	(25)	[3]	(84)	[3]
Natural Gas	(1)	[3]	(1)	[3]
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	63		226
Total assets	3,288		3,374
Total liabilities	129		147
Other Investments	71		84
Fair Value [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	19		38
U.S. equity securities	1,140		1,057
Non-U.S. equity securities	459		457
U.S. government and agency securities	905		916
Municipal securities	55		25
Other fixed-income securities	172		162
Total assets	2,750	[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	149		143
Electricity	142		139
Natural Gas	7		4
Electricity	126		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	113		109
Fixed-income securities	41		39
Life insurance contracts	72		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	5		9
U.S. equity securities	12		14
Non-U.S. equity securities	11		12
Total assets	142		157
Fixed-income securities	$ 114		$ 122

[1] Represents amount before deducting $322 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.