Regulatory - Additional Information (Detail) $ in Millions							3 Months Ended			9 Months Ended		12 Months Ended
	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, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Oct. 31, 2013 USD ($)
Public Utilities General Disclosures [Line Items]
Tax reform benefits							$ (23)	$ (31)		$ (63)	$ (65)
O&M expense							134	$ 153		396	472
Regulatory liability							1,311			1,311		$ 1,310
Amount refundable on settlement		$ 12
Regulatory asset for amortization										0	$ (80)
PGS [Member]
Public Utilities General Disclosures [Line Items]
Regulatory asset for amortization												11
Reduction in annual depreciation rates										10
Reduction in annual base rates										12
Hurricane Dorian [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration preparation costs							8
Tax Reform And Storm Settlement
Public Utilities General Disclosures [Line Items]
Tax reform benefits										103
Annually approved lowering base rates, Amount			$ 103
Tax Reform and Storm Settlement Two [Member]
Public Utilities General Disclosures [Line Items]
O&M expense												$ 47
Reduction in regulatory asset										47
Tax Reform and Storm Settlement Three [Member]
Public Utilities General Disclosures [Line Items]
O&M expense										56
Regulatory liability							$ 56			$ 56
Hurricane Irma [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs									$ 102
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] | 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	$ 27
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
Storm Restoration Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Minimum cost recovery period										12 months
Replenishment reserve for recovery of cost													$ 56