Regulatory - Additional Information (Detail) $ in Millions							3 Months Ended			6 Months Ended	9 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	Feb. 07, 2017	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 31, 2013 USD ($)
Public Utilities General Disclosures [Line Items]
Storm restoration costs			$ 10
Regulatory assets							$ 386			$ 386	$ 386				$ 433
Reduction in O&M expense for amortization										12
Annually approved lowering base rates, Amount	$ 103
Regulatory liabilities							1,298			1,298	1,298				1,285
Reduction in regulatory asset for amortization											$ (80)	$ 67
PGS [Member]
Public Utilities General Disclosures [Line Items]
Settlement agreement, approval date											Feb. 07, 2017
Regulatory liabilities														$ 5
Tax reform benefits							5
Reduction in regulatory asset for amortization							(11)				$ (10)
PGS [Member] | Scenario Forecast [Member]
Public Utilities General Disclosures [Line Items]
Reduction in annual depreciation rates													$ 10
Reduction in annual base rates													$ 12
PGS and OPC [Member]
Public Utilities General Disclosures [Line Items]
Regulatory assets							32			32	32
Amortization expenses											$ 21
Regulatory asset amortization beginning period						2016
Regulatory asset amortization ending period						2020
Tax Reform and Storm Settlement [Member]
Public Utilities General Disclosures [Line Items]
O&M expense and regulatory liability								$ 19		25
Hurricane Irma [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs									$ 102
Hurricane Irma [Member] | Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs									90
Hurricane Irma [Member] | O&M Expenses [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs									3
Hurricane Irma [Member] | Capital Expenditures [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs									$ 9
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
Cost cap of project | $ / kWac					1,500
Solar generation capacity investments					$ 850
Impact of tax reform through reduction in base revenue days					120 days
Solar Project Cost Recovery [Member] | Last Tranche [Member]
Public Utilities General Disclosures [Line Items]
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
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
Storm Restoration Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Minimum cost recovery period											12 months
Replenishment reserve for recovery of cost																$ 56
Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Regulatory assets							0			0	$ 0				$ 47
Regulatory liabilities							$ 56			56	$ 56
Tax Reform and Storm Settlement [Member]
Public Utilities General Disclosures [Line Items]
O&M expense and regulatory asset										$ 47