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
Significant Assumptions Used to Measure Costs [Abstract]
Liability, Defined Benefit Plan, Noncurrent	$ 161	$ 212
Defined Contribution Plan, Administrative Expense	12	11	$ 11
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at net asset value	61	68
Plan amendments	0	0
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	380
Service cost	1	1	1
Interest cost	13	15	15
Medicare subsidy reimbursements	1	1
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	6	7
Actuarial (gain) loss	52	12
Benefit payments	(38)	(34)
Obligation at Dec. 31	415	380
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	401
Actual return (loss) on plan assets	32	52
Plan participants' contributions	6	7
Employer contributions	3	4
Benefit payments	(38)	(34)
Fair value of plan assets at Dec. 31	404	401
Funded Status of Plans at Dec. 31 [Abstract]
Funded status	(11)	21
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]
Net loss	81	45
Prior service (credit) cost	6	10
Total	75	35
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Noncurrent regulatory assets	$ 75	$ 35
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Pre-65	5.50%	6.00%
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	13	15	15
Expected return on plan assets	(17)	(19)	(23)
Amortization of prior service (credit) cost	(4)	(5)	(6)
Amortization of net loss	1	3	4
Net periodic pension cost	$ (7)	$ (5)	$ (9)
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)	3.47%	4.32%	3.62%
Expected average long-term rate of return on assets (as a percent)	4.50%	4.50%	5.30%
Liability, Defined Benefit Plan, Noncurrent	$ 11	$ 21
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	$ (11)	$ 21
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	2.65%	3.47%
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Assets	$ 0	$ 0
Deferred Income Taxes	0	0
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Net-Of-Tax Accumulated Other Comprehensive Income	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	0	0	$ 0
Defined Benefit Plan, Costs Not Recognized Due To Regulation	3	1	2
Net benefit cost recognized for financial reporting	$ (4)	$ (4)	(7)
Target pension asset allocations (as a percent)	100.00%	100.00%
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	$ 31
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt, Next Twelve Months	2
Defined Benefit Plan Net Projected Benefit Payments In Year One	29
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	31
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt, Year Two	2
Defined Benefit Plan Net Projected Benefit Payments In Year Two	29
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	30
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt, Year Three	2
Defined Benefit Plan Net Projected Benefit Payments In Year Three	28
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	30
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt, Year Four	2
Defined Benefit Plan Net Projected Benefit Payments In Year Four	28
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	29
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt, Year Five	2
Defined Benefit Plan Net Projected Benefit Payments In Year Five	27
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	130
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt, after Year Five	12
Defined Benefit Plan Net Projected Benefit Payments In Five Fiscal Years Thereafter	$ 118
Other Postretirement Benefits Plan [Member] | Domestic and international equity securities
Significant Assumptions Used to Measure Costs [Abstract]
Target pension asset allocations (as a percent)	15.00%	15.00%
Other Postretirement Benefits Plan [Member] | Long-Duration Fixed Income and Interest Rate Swap Securities [Member]
Significant Assumptions Used to Measure Costs [Abstract]
Target pension asset allocations (as a percent)	0.00%	0.00%
Other Postretirement Benefits Plan [Member] | Short-to-intermediate fixed income securities
Significant Assumptions Used to Measure Costs [Abstract]
Target pension asset allocations (as a percent)	72.00%	72.00%
Other Postretirement Benefits Plan [Member] | Alternative investments
Significant Assumptions Used to Measure Costs [Abstract]
Target pension asset allocations (as a percent)	9.00%	9.00%
Other Postretirement Benefits Plan [Member] | Cash
Defined Benefit Plan Disclosure [Line Items]
Plan assets at net asset value	$ 0	$ 0
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	20
Fair value of plan assets at Dec. 31	$ 24	$ 20
Significant Assumptions Used to Measure Costs [Abstract]
Target pension asset allocations (as a percent)	4.00%	4.00%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at net asset value	$ 388	$ 348
Plan amendments	0	0
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	1,330
Service cost	30	26	29
Interest cost	46	52	47
Medicare subsidy reimbursements	0	0
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	0	0
Actuarial (gain) loss	102	108
Benefit payments	(80)	(85)
Obligation at Dec. 31	1,428	1,330
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	1,124
Actual return (loss) on plan assets	189	197
Plan participants' contributions	0	0
Employer contributions	50	46
Benefit payments	(80)	(85)
Fair value of plan assets at Dec. 31	1,283	1,124
Funded Status of Plans at Dec. 31 [Abstract]
Funded status	(145)	(206)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]
Net loss	432	482
Prior service (credit) cost	1	4
Total	431	478
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Noncurrent regulatory assets	404	452
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	30	26	29
Interest cost	46	52	47
Expected return on plan assets	(70)	(69)	(69)
Amortization of prior service (credit) cost	(3)	(3)	(3)
Amortization of net loss	30	25	31
Net periodic pension cost	$ 33	$ 34	$ 40
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)	3.49%	4.31%	3.63%
Expected average long-term rate of return on assets (as a percent)	6.84%	6.84%	6.84%
Liability, Defined Benefit Plan, Noncurrent	$ 145	$ 206
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	$ (145)	$ (206)
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	2.71%	3.49%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	3.75%	3.75%
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Assets	$ 25	$ 22
Deferred Income Taxes	1	1
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Net-Of-Tax Accumulated Other Comprehensive Income	1	3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	0	3	$ 5
Defined Benefit Plan, Costs Not Recognized Due To Regulation	4	4	(4)
Net benefit cost recognized for financial reporting	$ 37	$ 38	$ 36
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	3.75%	3.75%	3.75%
Target pension asset allocations (as a percent)	100.00%	100.00%
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	$ 83
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	83
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	82
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	82
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	82
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	393
Pension Plan [Member] | Domestic and international equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at net asset value	0	$ 0
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	30
Fair value of plan assets at Dec. 31	$ 27	$ 30
Significant Assumptions Used to Measure Costs [Abstract]
Target pension asset allocations (as a percent)	35.00%	37.00%
Pension Plan [Member] | Long-Duration Fixed Income and Interest Rate Swap Securities [Member]
Significant Assumptions Used to Measure Costs [Abstract]
Target pension asset allocations (as a percent)	35.00%	30.00%
Pension Plan [Member] | Short-to-intermediate fixed income securities
Significant Assumptions Used to Measure Costs [Abstract]
Target pension asset allocations (as a percent)	13.00%	14.00%
Pension Plan [Member] | Alternative investments
Significant Assumptions Used to Measure Costs [Abstract]
Target pension asset allocations (as a percent)	15.00%	17.00%
Pension Plan [Member] | Cash
Defined Benefit Plan Disclosure [Line Items]
Plan assets at net asset value	$ 0	$ 0
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	46
Fair value of plan assets at Dec. 31	$ 75	$ 46
Significant Assumptions Used to Measure Costs [Abstract]
Target pension asset allocations (as a percent)	2.00%	2.00%