Regulatory - Additional Information (Detail)									1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Aug. 20, 2018 USD ($)	Jun. 29, 2018 USD ($) MW	Dec. 28, 2017 USD ($)	Dec. 12, 2017 USD ($) MW	Sep. 27, 2017 USD ($) $ / kWac MW	Dec. 15, 2016 USD ($)	Jun. 28, 2016	Sep. 06, 2013	May 31, 2009	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2018 USD ($)	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%
Storm restoration costs			$ 10,000,000
Regulatory assets													$ 458,000,000	$ 433,000,000
Regulatory liability													1,310,000,000	1,285,000,000
Reduction in regulatory asset for amortization													(98,000,000)	100,000,000	$ 11,000,000
PGS [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE							10.75%		10.75%
Allowed equity in the capital structure									54.70%
Reduction in regulatory asset for amortization													(11,000,000)
PGS [Member] | Scenario Forecast [Member]
Public Utilities General Disclosures [Line Items]
Reduction in annual depreciation expense												$ 10,000,000
Reduction in annual base rates												$ 12,000,000
PGS and OPC [Member]
Public Utilities General Disclosures [Line Items]
Regulatory assets															$ 32,000,000
Reduction in annual depreciation expense						$ 16,000,000
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
Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Regulatory liability													$ 56,000,000	0
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
Minimum [Member] | PGS [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE									9.75%
Minimum [Member] | PGS and OPC [Member]
Public Utilities General Disclosures [Line Items]
Amortization period of Regulatory Asset															2 years
Decrease bottom return on equity							9.25%
Amortization expenses															$ 21,000,000
Maximum [Member] | PGS [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE							11.75%		11.75%
Maximum [Member] | PGS and OPC [Member]
Public Utilities General Disclosures [Line Items]
Decrease bottom return on equity							9.75%
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
Impact of tax reform through reduction in base revenue days					120 days
Investments in gas reserves					$ 0
Solar Project Cost Recovery [Member] | Minimum [Member]
Public Utilities General Disclosures [Line Items]
Solar project investment term					2017
Extended agreement base rate freeze date					Jan. 01, 2018
Solar Project Cost Recovery [Member] | Maximum [Member]
Public Utilities General Disclosures [Line Items]
Solar project investment term					2021
Extended agreement base rate freeze date					Dec. 31, 2021
Solar Project Cost Recovery [Member] | Effective September 1, 2018 [Member]
Public Utilities General Disclosures [Line Items]
Solar base rate adjustments					$ 31,000,000
Solar energy capacity | MW				145	150
Estimated annual revenue requirements				$ 24,000,000
Solar Project Cost Recovery [Member] | Effective January 1, 2019 [Member]
Public Utilities General Disclosures [Line Items]
Solar base rate adjustments					$ 51,000,000
Solar energy capacity | MW		260			250
Estimated annual revenue requirements		$ 46,000,000
Solar Project Cost Recovery [Member] | Effective January 1, 2020 [Member]
Public Utilities General Disclosures [Line Items]
Solar base rate adjustments					$ 31,000,000
Solar energy capacity | MW					150
Solar Project Cost Recovery [Member] | Effective January 1, 2021 [Member]
Public Utilities General Disclosures [Line Items]
Solar base rate adjustments					$ 10,000,000
Solar energy capacity | MW					50
Solar Project Cost Recovery [Member] | First Two Tranches [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW					400
Maximum cost to be constructed to build last tranche | $ / kWac					1,475
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													$ 3,000,000	$ 47,000,000
Regulatory liability													56,000,000
January 16, 2017 [Member]
Public Utilities General Disclosures [Line Items]
Additional base Revenue generated from increase in service charge													$ 110,000,000
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
Computer Software [Member]
Public Utilities General Disclosures [Line Items]
Amortization period of Regulatory Asset													15 years