Regulatory - Additional Information (Detail) $ in Millions											3 Months Ended			9 Months Ended		12 Months Ended
	Oct. 22, 2020 USD ($)	Aug. 18, 2020 USD ($)	Aug. 03, 2020 USD ($)	Apr. 27, 2020 USD ($)	Jun. 28, 2019 USD ($) MW	Apr. 09, 2019 USD ($)	Aug. 20, 2018 USD ($)	Jun. 29, 2018 USD ($) MW	Dec. 12, 2017 USD ($) MW	Sep. 27, 2017 USD ($) MW	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Jun. 09, 2020 USD ($)	Jun. 01, 2020 USD ($)	Oct. 31, 2013 USD ($)
Public Utilities General Disclosures [Line Items]
Federal statutory tax rate														4.50%		5.50%
Decrease in deferred income taxes offset to regulatory liability														$ 5
Utility plant, at original costs											$ 13,462			13,462		$ 12,590
Accumulated depreciation											3,654			3,654		3,472
Provision for income taxes											27	$ 23		67	$ 63
Amount refundable on settlement						$ 12
Increase in amortization of regulatory asset associated with environmental remediation costs														14	$ 0
Hurricane Irma [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs													$ 102
Hurricane Dorian [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration preparation costs																8
Tax Reform and Storm Settlement [Member]
Public Utilities General Disclosures [Line Items]
Provision for income taxes														103
Annually approved lowering base rates, Amount							$ 103
Intangible Software Assets [Member]
Public Utilities General Disclosures [Line Items]
Accumulated amortization reserve surplus																		$ 16
Recognition of accumulated amortization reserve surplus											4			12
Intangible Software Assets [Member] | Forecast [Member]
Public Utilities General Disclosures [Line Items]
Accumulated amortization reserve surplus																	$ 4
Electric Utility Plant [Member]
Public Utilities General Disclosures [Line Items]
Utility plant, at original costs											11,222			11,222		$ 10,578
Peoples Gas System [Member]
Public Utilities General Disclosures [Line Items]
Accumulated depreciation	$ 34
Reduction in annual base rates														12
Reduction in annual depreciation expense														10
Increase in annual base rate	58
Increase in revenue	$ 34
Allowed equity in the capital structure	54.70%
Impact of tax reform through reduction in base revenue days	120 days
Peoples Gas System [Member] | 2017 Settlement Agreement [Member]
Public Utilities General Disclosures [Line Items]
Increase in amortization of regulatory asset associated with environmental remediation costs														32
Reduction in annual depreciation expense														$ 16
Regulatory asset amortization beginning period														2016
Regulatory asset amortization ending period														2020
Peoples Gas System [Member] | Cast Iron and Bare Steel Replacement Rider [Member]
Public Utilities General Disclosures [Line Items]
Increase in revenue	$ 24
Minimum [Member] | Peoples Gas System [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE	8.90%
Base rate agreement frozen period	Jan. 01, 2021
Minimum [Member] | Peoples Gas System [Member] | 2017 Settlement Agreement [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE														9.25%
Maximum [Member] | Peoples Gas System [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE	11.00%
Base rate agreement frozen period	Dec. 31, 2023
Maximum [Member] | Peoples Gas System [Member] | 2017 Settlement Agreement [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE														11.75%
Mid Point [Member] | Peoples Gas System [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE	9.90%
Solar Project Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Settlement agreement, extended terms										four years through December 31, 2021
Settlement agreement, approval date										Nov. 06, 2017
Solar energy capacity | MW										600
Solar generation capacity investments										$ 850
Solar Project Cost Recovery [Member] | Solar Base Rate Adjustments [Member]
Public Utilities General Disclosures [Line Items]
True-up amount returned to customers		$ 5
Solar Project Cost Recovery [Member] | Effective September 1, 2018 [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW									145
Estimated annual revenue requirements									$ 24
Solar Project Cost Recovery [Member] | Effective January 1, 2019 [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW								260
Estimated annual revenue requirements								$ 46
Solar Project Cost Recovery [Member] | Effective January 1, 2020 [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW					149
Estimated annual revenue requirements					$ 26
Solar Project Cost Recovery [Member] | Effective January 1, 2021 [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW									46
Estimated annual revenue requirements									$ 8
Solar Project Cost Recovery [Member] | Minimum [Member]
Public Utilities General Disclosures [Line Items]
Solar project investment term										2017
Solar Project Cost Recovery [Member] | Maximum [Member]
Public Utilities General Disclosures [Line Items]
Solar project investment term										2021
Big Bend Modernization Project [Member]
Public Utilities General Disclosures [Line Items]
Energy generation capacity investments											850			$ 850
Investments in energy projects till date											$ 461			$ 461
Accumulated depreciation																			$ 90
Big Bend Modernization Project [Member] | Electric Utility Plant [Member]
Public Utilities General Disclosures [Line Items]
Utility plant, at original costs																			$ 223
Big Bend Modernization Project [Member] | Minimum [Member]
Public Utilities General Disclosures [Line Items]
Energy project investment term														2018
Big Bend Modernization Project [Member] | Maximum [Member]
Public Utilities General Disclosures [Line Items]
Energy project investment term														2023
Storm Protection Plan [Member]
Public Utilities General Disclosures [Line Items]
Reduction in annual base rates				$ 15
Cost recovery from settlement agreement			$ 39
Storm Restoration Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Minimum cost recovery period														12 months
Replenishment reserve for recovery of cost																				$ 56