New and Significant Accounting Policies (Reclassifications Out of Accumulated Other Comprehensived Income) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance		$ (9)
Change in investments		0	$ 0	$ (17)
Total other comprehensive income (loss)		1	(2)	(18)
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance		(8)	(9)
Net actuarial loss tax		0	1	0
Change in investments tax		0	0	12
Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss		(15)	(16)
Unrecognized prior service cost		5	14
Transfer to regulatory account		19	29
Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account		468	(7)
Amortization of prior service cost		6	33
Amortization of net actuarial loss		474	(43)
Pension Plans Defined Benefit [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance		(25)
Unrecognized net actuarial loss		5
Unrecognized prior service cost		(6)
Amortization of prior service cost		(7)	8	15
Amortization of net actuarial loss		22	24	10
Total other comprehensive income (loss)		0	(2)
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance		(25)	(25)
Pension Plans Defined Benefit [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss	[1]	(13)	(14)
Unrecognized prior service cost	[1]	(4)	5
Transfer to regulatory account	[1]	9	18
Net actuarial loss tax		9	10
Transfer To Regulatory Account Tax		6	13
Amortization of prior service cost tax		3	3
Pension Plans Defined Benefit [Member] | Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account		327	(7)
Amortization of prior service cost		6	54
Amortization of net actuarial loss		333	(64)
Net actuarial loss tax		229	45
Transfer To Regulatory Account Tax		225	5
Amortization of prior service cost tax		4	37
Other Benefits [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance		16
Unrecognized net actuarial loss		(5)
Unrecognized prior service cost		14
Amortization of prior service cost		15	15	19
Amortization of net actuarial loss		4	4	$ 4
Total other comprehensive income (loss)		1	0
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance		17	16
Other Benefits [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss		(2)	(2)
Unrecognized prior service cost		9	9
Transfer to regulatory account		10	11
Net actuarial loss tax		2	2
Transfer To Regulatory Account Tax		8	8
Amortization of prior service cost tax		6	6
Other Benefits [Member] | Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account		141	0
Amortization of prior service cost		0	21
Amortization of net actuarial loss		141	21
Net actuarial loss tax		97	15
Transfer To Regulatory Account Tax		97	0
Amortization of prior service cost tax		$ 0	$ 15

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