Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount primarily related to deferred taxes on appreciation of investment value	$ 347		$ 324
Fair Value Measurements, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	96		94
Total assets	2,423		2,478
Total liabilities	59		47
Other Investments			33
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	29		17
Total assets	2,321	[1]	2,343	[1]
Fixed-income securities	673		741
Global equity securities	1,619		1,585
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	2		1
Natural Gas	2		1
Electricity	59		47
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	4		7
Total assets	4		7
Fair Value Measurements, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	823		687
Total liabilities	12		8
Fair Value Measurements, Level 2 [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	552	[1]	402	[1]
Fixed-income securities	539		389
Global equity securities	13		13
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	8		18
Electricity	7		17
Natural Gas	1		1
Electricity	11		5
Natural Gas	1		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	116		114
Fixed-income securities	42		42
Life insurance contracts	74		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]
Total assets	147		153
Fixed-income securities	123		128
Global equity securities	24		25
Fair Value Measurements, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	230		232
Total liabilities	188		163
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	230		232
Electricity	230		232
Electricity	188		163
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	96		94
Total assets	3,485		3,399
Total liabilities	189		166
Other Investments			33
Fair Value [Member] | Nuclear Decommissioning Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market investments	29		17
Total assets	2,873	[1]	2,745	[1]
Fixed-income securities	1,212		1,130
Global equity securities	1,632		1,598
Fair Value [Member] | Price Risk Management Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	249		253
Electricity	246		251
Natural Gas	3		2
Electricity	188		163
Natural Gas	1		3
Fair Value [Member] | Rabbi Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	116		114
Fixed-income securities	42		42
Life insurance contracts	74		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	4		7
Total assets	151		160
Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	9	[2]	2	[2]
Total liabilities	(70)	[2]	(52)	[2]
Netting [Member] | Price Risk Management Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	9	[2]	2	[2]
Electricity	9	[2]	2	[2]
Electricity	$ (70)	[2]	$ (52)	[2]

[1] Represents amounts before deducting $347 million and $324 million at March 31, 2015 and December 31, 2014, 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.