New and Significant Accounting Policies (Reclassifications Out of Accumulated Other Comprehensived Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance	$ (7)
Change in investments	0	$ (17)	$ (25)
Total other comprehensive income (loss)	(2)	(18)	(39)
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance	(9)	(7)
Net actuarial loss tax	1	0	10
Change in investments tax	0	12	17
Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss	(16)	(9)
Unrecognized prior service cost	14	19
Transfer to regulatory account	29	26
Realized gain on investments		(17)
Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized prior service cost	33
Transfer to regulatory account	(7)	(104)
Amortization of net actuarial loss	(43)	(107)
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	4
Unrecognized prior service cost	15
Amortization of prior service cost	15	19	23
Amortization of net actuarial loss	4	4	$ 2
Total other comprehensive income (loss)	0	1
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance	16	16
Other Benefits [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss	(2)	(3)
Unrecognized prior service cost	9	11
Transfer to regulatory account	11	13
Realized gain on investments		0
Net actuarial loss tax	2	1
Transfer To Regulatory Account Tax	8	9
Amortization of prior service cost tax	6	8
Realized gain on investments tax		0
Other Benefits [Member] | Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized prior service cost	(21)
Transfer to regulatory account	0	(31)
Amortization of net actuarial loss	21	(31)
Net actuarial loss tax	15	21
Transfer To Regulatory Account Tax	0	21
Amortization of prior service cost tax	15
Other Investments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance	0
Total other comprehensive income (loss)		(17)
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance		0
Other Investments [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss		0
Unrecognized prior service cost		0
Transfer to regulatory account		0
Realized gain on investments		(17)
Net actuarial loss tax		0
Transfer To Regulatory Account Tax		0
Amortization of prior service cost tax		0
Realized gain on investments tax		12
Other Investments [Member] | Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account		0
Amortization of net actuarial loss		0
Net actuarial loss tax		0
Transfer To Regulatory Account Tax		0
Pension [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance	(23)
Total other comprehensive income (loss)	(2)	(2)
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance	(25)	(23)
Pension [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss	(14)	(6)
Unrecognized prior service cost	5	8
Transfer to regulatory account	18	13
Realized gain on investments		0
Net actuarial loss tax	10	4
Transfer To Regulatory Account Tax	13	10
Amortization of prior service cost tax	3	7
Realized gain on investments tax		0
Pension [Member] | Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized prior service cost	54
Transfer to regulatory account	(7)	(73)
Amortization of net actuarial loss	(64)	(76)
Net actuarial loss tax	45	51
Transfer To Regulatory Account Tax	5	$ 51
Amortization of prior service cost tax	$ 37