New and Significant Accounting Policies (Reclassifications Out of Accumulated Other Comprehensived Income) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance		$ 11	$ 50
Change in investments		(17)	(25)	$ 38
Total other comprehensive income (loss)		(18)	(39)	151
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance		(7)	11	50
Net actuarial loss tax		0	10	80
Change in investments tax		12	17	26
Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account	[1]	(26)	(28)
Amortization of prior service cost	[1]	19	26
Amortization of net actuarial loss	[1]	9	2
Realized gain on investments		(17)	(30)
Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss		(107)	(616)
Unrecognized prior service cost			1
Transfer to regulatory account		104	601
Change in investments			5
Other Benefits [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance		15	15
Unrecognized net actuarial loss		4
Unrecognized prior service cost		15
Amortization of prior service cost		19	23	23
Amortization of net actuarial loss		4	2	6
Total other comprehensive income (loss)		1	0
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance		16	15	15
Other Benefits [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account	[1]	(13)	(15)
Amortization of prior service cost	[1]	11	14
Amortization of net actuarial loss	[1]	3	1
Realized gain on investments		0	0
Net actuarial loss tax		1	1
Transfer To Regulatory Account Tax		9	10
Amortization of prior service cost tax		8	9
Realized gain on investments tax		0	0
Other Benefits [Member] | Other Comprehensive Income Before Reclassifications [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrecognized net actuarial loss		(31)	(28)
Unrecognized prior service cost			0
Transfer to regulatory account		31	28
Change in investments			0
Net actuarial loss tax		21	19
Transfer To Regulatory Account Tax		21	19
Change in investments tax			0
Amortization of prior service cost tax			0
Other Investments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance		17	42
Total other comprehensive income (loss)		(17)	(25)
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance		0	17	42
Other Investments [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account	[1]	0	0
Amortization of prior service cost	[1]	0	0
Amortization of net actuarial loss	[1]	0	0
Realized gain on investments		(17)	(30)
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		12	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
Transfer to regulatory account		0	0
Change in investments			5
Net actuarial loss tax		0	0
Transfer To Regulatory Account Tax		0	0
Change in investments tax			4
Amortization of prior service cost tax			0
Pension [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance		(21)	(7)
Total other comprehensive income (loss)		(2)	(14)
Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance		(23)	(21)	$ (7)
Realized gain on investments tax		0
Pension [Member] | Amounts Reclassified From Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Transfer to regulatory account	[1]	(13)	(13)
Amortization of prior service cost	[1]	8	12
Amortization of net actuarial loss	[1]	6	1
Realized gain on investments		0	0
Net actuarial loss tax		4	1
Transfer To Regulatory Account Tax		10	9
Amortization of prior service cost tax		7	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		(76)	(588)
Unrecognized prior service cost			1
Transfer to regulatory account		73	573
Change in investments			0
Net actuarial loss tax		51	404
Transfer To Regulatory Account Tax		$ 51	394
Change in investments tax			0
Amortization of prior service cost tax			$ 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.)