Regulatory - Additional Information (Detail)													1 Months Ended	3 Months Ended	12 Months Ended
	Nov. 19, 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. 28, 2017 USD ($)	Dec. 12, 2017 USD ($) MW	Feb. 07, 2017	Sep. 06, 2013	May 31, 2009	Sep. 30, 2017 USD ($)	Dec. 31, 2020 USD ($) $ / kWac	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 31, 2022 USD ($)	Feb. 01, 2021 USD ($)	Jun. 09, 2020 USD ($)	Jun. 01, 2020 USD ($)	Oct. 31, 2013 USD ($)
Public Utilities General Disclosures [Line Items]
Percentage of ROE															10.25%
Return on equity range															range of plus or minus 1%
Percentage change in ROE Percentage															1.00%
Allowed equity in the capital structure												54.00%
Federal statutory tax rate															4.50%	5.50%
Decrease in deferred income taxes offset to regulatory liability															$ 5,000,000
Utility plant, at original costs															13,818,000,000	$ 12,590,000,000
Accumulated depreciation															3,712,000,000	3,472,000,000
Storm restoration costs									$ 10,000,000
Regulatory assets															485,000,000	437,000,000
O&M expense															542,000,000	543,000,000	$ 632,000,000
Regulatory liability															1,261,000,000	1,303,000,000
Provision for income taxes															82,000,000	77,000,000	81,000,000
Amount refundable on settlement						$ 12,000,000
Reduction in regulatory asset for amortization															18,000,000	(1,000,000)	(98,000,000)
Peoples Gas System [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE											10.75%
Allowed equity in the capital structure	54.70%												54.70%
Accumulated depreciation	$ 34,000,000
Reduction in annual base rates																12,000,000
Reduction in regulatory asset for amortization																	(11,000,000)
Reduction in annual depreciation rates																10,000,000
Increase in annual base rate	58,000,000
Increase in revenue	$ 34,000,000
Impact of tax reform through reduction in base revenue days	120 days
PGS and OPC [Member]
Public Utilities General Disclosures [Line Items]
Regulatory assets																		$ 32,000,000
Date new bottom of return on equity range will remain in effect																			Dec. 31, 2020
Amortization expenses																		$ 5,000,000	$ 16,000,000
Regulatory asset amortization beginning period																		2016
Regulatory asset amortization ending period																		2020
Hurricane Dorian [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration preparation costs																8,000,000
Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Regulatory liability															48,000,000	48,000,000
Hurricane Irma [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs														$ 102,000,000
Hurricane Irma [Member] | Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs														90,000,000
Hurricane Irma [Member] | O&M Expenses [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs														3,000,000
Hurricane Irma [Member] | Capital Expenditures [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs														$ 9,000,000
Intangible Software Assets [Member]
Public Utilities General Disclosures [Line Items]
Accumulated amortization reserve surplus																						$ 16,000,000
Electric Utility Plant [Member]
Public Utilities General Disclosures [Line Items]
Utility plant, at original costs															$ 11,486,000,000	10,578,000,000
Cast Iron and Bare Steel Replacement Rider [Member] | Peoples Gas System [Member]
Public Utilities General Disclosures [Line Items]
Increase in revenue	$ 24,000,000
Scenario Forecast [Member]
Public Utilities General Disclosures [Line Items]
Revenue requirements																				$ 295,000,000
Subsequent Event [Member]
Public Utilities General Disclosures [Line Items]
Revenue requirements																					$ 280,000,000
Base rate adjustment amount																					$ 130,000,000
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] | PGS and OPC [Member]
Public Utilities General Disclosures [Line Items]
Decrease bottom return on equity											11.75%
Amortization expenses																			$ 21,000,000
Amortization period of Regulatory Asset																		2 years
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] | PGS and OPC [Member]
Public Utilities General Disclosures [Line Items]
Decrease bottom return on equity											9.25%
Mid Point [Member] | Peoples Gas System [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE	9.90%
Condition One [Member]
Public Utilities General Disclosures [Line Items]
ROE lower range limit															9.25%
Condition Two [Member]
Public Utilities General Disclosures [Line Items]
ROE lower range limit															11.25%
Tax Reform and Storm Settlement Two [Member]
Public Utilities General Disclosures [Line Items]
O&M expense																	47,000,000
Reduction in regulatory asset																	47,000,000
Tax Reform and Storm Settlement Three [Member]
Public Utilities General Disclosures [Line Items]
O&M expense																	56,000,000
Regulatory liability																	$ 56,000,000
Tax Reform and Storm Settlement [Member]
Public Utilities General Disclosures [Line Items]
Annually approved lowering base rates, Amount							$ 103,000,000
Provision for income taxes															$ 103,000,000
Solar Project Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Settlement agreement, extended terms															four years through 2021
Settlement agreement, approval date															Nov. 06, 2017
Cost cap of project | $ / kWac															1,500
Cost savings benefit percentage for projects below cost cap															75.00%
Solar generation capacity investments															$ 850,000,000
Investments in gas reserves															$ 0
Solar Project Cost Recovery [Member] | Solar Base Rate Adjustments [Member]
Public Utilities General Disclosures [Line Items]
True-up amount returned to customers		$ 5,000,000
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
Solar Project Cost Recovery [Member] | Invested Through December 31, 2019 [Member]
Public Utilities General Disclosures [Line Items]
Solar generation capacity investments																$ 820,000,000
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										$ 8,000,000
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										$ 24,000,000
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								$ 46,000,000
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					$ 26,000,000
Big Bend Modernization Project [Member]
Public Utilities General Disclosures [Line Items]
Energy generation capacity investments															$ 850,000,000
Investments in energy projects till date															526,000,000
Accumulated depreciation															88,000,000								$ 90,000,000
Big Bend Modernization Project [Member] | Electric Utility Plant [Member]
Public Utilities General Disclosures [Line Items]
Utility plant, at original costs															$ 200,000,000								$ 223,000,000
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				$ 15,000,000
Cost recovery from settlement agreement			$ 39,000,000
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
Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Regulatory assets									$ 56,000,000