New and Significant Accounting Policies (Reclassifications Out of Accumulated Other Comprehensived Income) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance	$ 50		$ (101)
Change in investments	(25)		38		4
Total other comprehensive income (loss)	(39)		151		112
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance	11		50		(101)
Net actuarial loss tax	10		80		72
Change in investments tax	17		26		3
Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account	(28)	[1]	(76)	[1]
Amortization of prior service cost	26	[1]	25	[1]
Amortization of net actuarial loss	2	[1]	69	[1]
Realized gain on investments	(30)
Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss	(616)		1,214
Unrecognized prior service cost	1		1,119
Transfer to regulatory account	601
Change in investments	5		38
Other Benefits [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance	15		(77)
Unrecognized net actuarial loss	4
Unrecognized prior service cost	19
Amortization of prior service cost	23		23		25
Amortization of net actuarial loss	2		6		6
Total other comprehensive income (loss)	0
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance	15		15		(77)
Other Benefits [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account	(15)	[1]	0	[1]
Amortization of prior service cost	14	[1]	13	[1]
Amortization of net actuarial loss	1	[1]	3	[1]
Realized gain on investments	0
Total other comprehensive income (loss)			92
Net actuarial loss tax	1		3
Transfer To Regulatory Account Tax	10		0
Amortization of prior service cost tax	9		10
Realized gain on investments tax	0
Other Benefits [Member] | Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss	(28)		45
Unrecognized prior service cost	0		(31)
Transfer to regulatory account	28
Change in investments	0		0
Net actuarial loss tax	19		35
Transfer To Regulatory Account Tax	19		22
Change in investments tax	0		0
Other Investments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance	42
Total other comprehensive income (loss)	(25)
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance	17				4
Other Investments [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account	0	[1]	0	[1]
Amortization of prior service cost	0	[1]	0	[1]
Amortization of net actuarial loss	0	[1]	0	[1]
Realized gain on investments	(30)
Total other comprehensive income (loss)			38
Net actuarial loss tax	0		0
Transfer To Regulatory Account Tax	0		0
Amortization of prior service cost tax	0		0
Realized gain on investments tax	20
Other Investments [Member] | Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss	0		0
Unrecognized prior service cost	0		0
Transfer to regulatory account	0
Change in investments	5		38
Net actuarial loss tax	0		0
Transfer To Regulatory Account Tax	0		0
Change in investments tax	4		26
Pension [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance	(7)
Total other comprehensive income (loss)	(14)
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance	(21)				(28)
Pension [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account	(13)	[1]	(76)	[1]
Amortization of prior service cost	12	[1]	12	[1]
Amortization of net actuarial loss	1	[1]	66	[1]
Realized gain on investments	0
Total other comprehensive income (loss)			21
Net actuarial loss tax	1		45
Transfer To Regulatory Account Tax	9		54
Amortization of prior service cost tax	8		8
Realized gain on investments tax	0
Pension [Member] | Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss	(588)		1,169
Unrecognized prior service cost	1		1,150
Transfer to regulatory account	573
Change in investments	0		0
Net actuarial loss tax	404		804
Transfer To Regulatory Account Tax	394		790
Change in investments tax	$ 0		$ 0

[1] These components are included in the computation of net periodic pension and other postretirement benefit costs. (See Note 11 below for additional details.)