Regulatory - Additional Information (Detail) - USD ($)						1 Months Ended						9 Months Ended	12 Months Ended
	Nov. 09, 2023	Jan. 23, 2023	Jul. 01, 2022	Aug. 06, 2021	Oct. 31, 2013	Jan. 31, 2024	Sep. 30, 2023	Jan. 31, 2023	Nov. 30, 2022	Sep. 30, 2022	Jan. 31, 2022	Sep. 30, 2023	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2022
Public Utilities General Disclosures [Line Items]
Base rate adjustment amount			$ 10,000,000
Adjustments to fuel charges recovery		$ 518,000,000
Adjustments to fuel charges recovery projected reduction		$ 170,000,000
Accumulated depreciation							$ 3,367,000,000					$ 3,367,000,000				$ 3,845,000,000
Additional cost recovery from settlement agreement											$ 169,000,000
Settlement Agreement [Member]
Public Utilities General Disclosures [Line Items]
Base rate adjustment amount				$ 123,000,000
Increase in revenue				$ 191,000,000
Settlement agreement number of subsequent years adjustments				2 years
Settlement agreement adjustment amount								$ 90,000,000
Allowed equity in the capital structure				54.00%
Basis point increase applicable				0.25%
Additional attainable revenue under settlement agreement				$ 10,000,000
Changes in base rate under agreement				$ 0
Impact of tax reform through reduction in base revenue days				180 days
Settlement Agreement [Member] | Scenario Forecast [Member]
Public Utilities General Disclosures [Line Items]
Settlement agreement adjustment amount						$ 21,000,000
Settlement Agreement [Member] | US Treasury Bond [Member]
Public Utilities General Disclosures [Line Items]
Calculating period				30 years
Determining period				6 months
Minimum [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE			9.25%
Minimum [Member] | Settlement Agreement [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE				9.00%
Base rate agreement frozen period				Jan. 01, 2022
Minimum [Member] | Settlement Agreement [Member] | US Treasury Bond [Member]
Public Utilities General Disclosures [Line Items]
ROE revenue basis point on yield rate				0.50%
Maximum [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE			11.25%
Maximum [Member] | Settlement Agreement [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE				11.00%
Base rate agreement frozen period				Dec. 31, 2024
Mid Point [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE			10.20%
Mid Point [Member] | Settlement Agreement [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE				9.95%
Retiring Assets [Member] | Settlement Agreement [Member]
Public Utilities General Disclosures [Line Items]
Cost recovery from settlement agreement				$ 68,000,000
Retiring Coal Generation Units and Meter Assets [Member] | Settlement Agreement [Member]
Public Utilities General Disclosures [Line Items]
Settlement agreement cost recovery period				15 years
Storm Restoration Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Minimum cost recovery period												12 months
Replenishment reserve for recovery of cost					$ 56,000,000
Multiple Hurricanes [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration preparation costs													$ 10,000,000	$ 10,000,000	$ 10,000,000
Hurricane Ian [Member]
Public Utilities General Disclosures [Line Items]
Restoration costs										$ 129,000,000
Storm reserve expected to charge												$ 121,000,000
Storm restoration costs charged to reserve												10,000,000
Replenishment of balance in reserve					$ 131,000,000							$ 56,000,000
Hurricane Ian [Member] | Subsequent Event [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs	$ 134,000,000
Storm restoration costs balance	$ 29,000,000
Hurricane Nicole [Member]
Public Utilities General Disclosures [Line Items]
Restoration costs									$ 2,000,000
Hurricane Idalia [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs							$ 36,000,000