Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) Plan	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement			$ 7,000,000
Operating and maintenance expenses		$ 275,000,000	285,000,000	$ 283,000,000
Significant Assumptions Used to Measure Costs [Abstract]
Liability, Defined Benefit Plan, Noncurrent		(42,000,000)	(67,000,000)
annual interest crediting rates		2.37	3.12	3.75
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement			2,000,000	3,000,000
Accumulated Benefit Obligation at Dec. 31		519,000,000	481,000,000
Operating and maintenance expenses			1,000,000	1,000,000
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	$ 562,000,000	519,000,000	478,000,000
Service cost		10,000,000	9,000,000	10,000,000
Interest cost		18,000,000	20,000,000	18,000,000
Plan amendments		0	0
Actuarial loss		45,000,000	44,000,000
Benefit payments		(30,000,000)	(32,000,000)
Obligation at Dec. 31		562,000,000	519,000,000	478,000,000
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	527,000,000	458,000,000	392,000,000
Actual return on plan assets		84,000,000	80,000,000
Employer contributions		15,000,000	18,000,000
Benefit payments		(30,000,000)	(32,000,000)
Fair value of plan assets at Dec. 31		527,000,000	458,000,000	392,000,000
Funded Status of Plans at Dec. 31 [Abstract]
Funded status		(35,000,000)	(61,000,000)
Assets for Plan Benefits, Defined Benefit Plan		0	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		186,000,000	210,000,000
Prior service credit		(1,000,000)	(1,000,000)
Total		185,000,000	209,000,000
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Current regulatory assets		11,000,000	11,000,000
Noncurrent regulatory assets		174,000,000	198,000,000
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Liabilities		0	0
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Noncurrent Regulatory Liabilities		0	0
Total		$ 185,000,000	$ 209,000,000
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)		2.71%	3.49%
Expected average long-term increase in compensation level (as a percent)		3.75%	3.75%
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		$ 10,000,000	$ 9,000,000	10,000,000
Interest cost		18,000,000	20,000,000	18,000,000
Expected return on plan assets		(29,000,000)	(28,000,000)	(28,000,000)
Amortization of prior service cost		0	0	0
Amortization of net loss		12,000,000	11,000,000	14,000,000
Net periodic pension cost		11,000,000	14,000,000	17,000,000
Net benefit cost recognized for financial reporting		$ 13,000,000	$ 15,000,000	$ 15,000,000
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%
Plan participant's contributions		$ 0	$ 0
Plan participant's contributions		0	0
Liability, Defined Benefit Plan, Noncurrent		(35,000,000)	(61,000,000)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position		(35,000,000)	(61,000,000)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement		0	2,000,000	$ 3,000,000
Defined Benefit Plan, Costs Not Recognized Due To Regulation		2,000,000	1,000,000	(2,000,000)
Pension Plan [Member] | SPS [Member]
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		458,000,000
Fair value of plan assets at Dec. 31			458,000,000
Cash Flows [Abstract]
Payment for Pension Benefits		14,000,000	18,000,000	8,000,000
Other Postretirement Benefits Plan [Member]
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	38,000,000	44,000,000	42,000,000
Service cost		1,000,000	1,000,000	1,000,000
Interest cost		1,000,000	2,000,000	2,000,000
Plan amendments		0	0
Actuarial loss		(5,000,000)	0
Benefit payments		(4,000,000)	(2,000,000)
Obligation at Dec. 31		38,000,000	44,000,000	42,000,000
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	44,000,000	44,000,000	40,000,000
Actual return on plan assets		3,000,000	5,000,000
Employer contributions		0	0
Benefit payments		(4,000,000)	(2,000,000)
Fair value of plan assets at Dec. 31		44,000,000	44,000,000	40,000,000
Funded Status of Plans at Dec. 31 [Abstract]
Funded status		6,000,000	0
Assets for Plan Benefits, Defined Benefit Plan		6,000,000	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		(18,000,000)	(12,000,000)
Prior service credit		(1,000,000)	(1,000,000)
Total		(19,000,000)	(13,000,000)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Current regulatory assets		0	0
Noncurrent regulatory assets		0	0
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Liabilities		(1,000,000)	(1,000,000)
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Noncurrent Regulatory Liabilities		(18,000,000)	(12,000,000)
Total		$ (19,000,000)	$ (13,000,000)
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)		2.65%	3.47%
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		$ 1,000,000	$ 1,000,000	1,000,000
Interest cost		1,000,000	2,000,000	2,000,000
Expected return on plan assets		(2,000,000)	(2,000,000)	(3,000,000)
Amortization of prior service cost		0	(1,000,000)	0
Amortization of net loss		0	0	0
Net periodic pension cost		0	0	0
Net benefit cost recognized for financial reporting		$ 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%
Plan participant's contributions		$ 1,000,000	$ 1,000,000
Plan participant's contributions		1,000,000	1,000,000
Liability, Defined Benefit Plan, Noncurrent		0	0
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position		$ 6,000,000	$ 0
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
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		0	0	$ 0
Other Postretirement Benefits Plan [Member] | SPS [Member]
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		$ 44,000,000
Fair value of plan assets at Dec. 31			$ 44,000,000
Subsequent Event | Pension Plan [Member] | SPS [Member]
Cash Flows [Abstract]
Payment for Pension Benefits	$ 14,000,000
Parent Company [Member] | Pension Plan [Member]
Cash Flows [Abstract]
Number of pension plans to which contributions were made | Plan		4