Regulatory - Additional Information (Detail) $ in Millions						3 Months Ended			12 Months Ended
	Apr. 09, 2019 USD ($)	Aug. 20, 2018 USD ($)	Jun. 29, 2018 USD ($) MW	Dec. 12, 2017 USD ($) MW	Sep. 27, 2017 USD ($) MW	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($)
Public Utilities General Disclosures [Line Items]
Tax reform benefits						$ (16)	$ (14)
Regulatory liability						1,324			$ 1,310
Regulatory asset for amortization						4	$ (20)
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
Subsequent Event [Member]
Public Utilities General Disclosures [Line Items]
Amount refundable on settlement	$ 12
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
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 generation capacity investments					$ 850
Solar energy capacity | MW					600
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] | 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