Benefit Plans and Other Postretirement Benefits, Postretirement Benefit Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement		$ 7
Significant Assumptions Used to Measure Costs [Abstract]
Liability, Defined Benefit Plan, Noncurrent	$ (42)	(67)
Operating and maintenance expenses	275	285	$ 283
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan amendments	0	0
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	44	42
Service cost	1	1	1
Interest cost	1	2	2
Plan participant's contributions	1	1
Actuarial loss	(5)	0
Benefit payments	(4)	(2)
Obligation at Dec. 31	38	44	42
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	44	40
Actual return on plan assets	3	5
Plan participant's contributions	1	1
Employer contributions	0	0
Benefit payments	(4)	(2)
Fair value of plan assets at Dec. 31	44	44	40
Funded Status of Plans at Dec. 31 [Abstract]
Funded status	6	0
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery Rates [Abstract]
Net loss	(18)	(12)
Prior service credit	(1)	(1)
Total	(19)	(13)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Credit) Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Assets	0	0
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Noncurrent Regulatory Assets	0	0
Current regulatory liabilities	(1)	(1)
Noncurrent regulatory liabilities	(18)	(12)
Total	$ (19)	$ (13)
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)	2.65%	3.47%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Pre-65	5.50%	6.00%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Post-65	5.00%	5.10%
Ultimate health care trend assumption rate (as a percent)	4.50%	4.50%
Period until ultimate trend rate is reached (in years)	5	3
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 1	$ 1	1
Interest cost	1	2	2
Expected return on plan assets	(2)	(2)	(3)
Amortization of prior service cost	0	(1)	0
Amortization of net loss	0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	0	0	0
Net periodic pension cost	$ 0	$ 0	$ 0
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)	3.47%	4.32%	3.62%
Expected average long-term increase in compensation level (as a percent)	0.00%	0.00%	0.00%
Expected average long-term rate of return on assets (as a percent)	4.50%	5.30%	5.80%
Liability, Defined Benefit Plan, Noncurrent	$ 0	$ 0
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	6	0
Defined Benefit Plan, Costs Not Recognized Due To Regulation	0	0	$ 0
Net benefit cost recognized for financial reporting	0	0	0
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement		2	3
Plan amendments	0	0
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	519	478
Service cost	10	9	10
Interest cost	18	20	18
Plan participant's contributions	0	0
Actuarial loss	45	44
Benefit payments	(30)	(32)
Obligation at Dec. 31	562	519	478
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	458	392
Actual return on plan assets	84	80
Plan participant's contributions	0	0
Employer contributions	15	18
Benefit payments	(30)	(32)
Fair value of plan assets at Dec. 31	527	458	392
Funded Status of Plans at Dec. 31 [Abstract]
Funded status	(35)	(61)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery Rates [Abstract]
Net loss	186	210
Prior service credit	(1)	(1)
Total	185	209
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Credit) Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Assets	11	11
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Noncurrent Regulatory Assets	174	198
Current regulatory liabilities	0	0
Noncurrent regulatory liabilities	0	0
Total	$ 185	$ 209
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)	2.71%	3.49%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	3.75%	3.75%
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 10	$ 9	10
Interest cost	18	20	18
Expected return on plan assets	(29)	(28)	(28)
Amortization of prior service cost	0	0	0
Amortization of net loss	12	11	14
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	0	(2)	(3)
Net periodic pension cost	$ 11	$ 14	$ 17
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)	3.49%	4.31%	3.63%
Expected average long-term increase in compensation level (as a percent)	3.75%	3.75%	3.75%
Expected average long-term rate of return on assets (as a percent)	6.78%	6.78%	6.78%
Liability, Defined Benefit Plan, Noncurrent	$ (35)	$ (61)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	(35)	(61)
Defined Benefit Plan, Costs Not Recognized Due To Regulation	(2)	(1)	$ 2
Net benefit cost recognized for financial reporting	$ 13	15	15
Operating and maintenance expenses		$ 1	$ 1