Regulatory - Additional Information (Detail) $ in Millions			3 Months Ended
	Dec. 12, 2017 USD ($) MW	Sep. 27, 2017 USD ($) $ / kWac MW	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Oct. 31, 2013 USD ($)
Public Utilities General Disclosures [Line Items]
Storm damage reserve						$ 56
Regulatory assets			$ 418		$ 433
Regulatory liabilities			$ 1,285		1,285
PGS [Member]
Public Utilities General Disclosures [Line Items]
Settlement agreement, approval date			Feb. 07, 2017
Regulatory liabilities			$ 2
Tax Reform And Storm Settlement
Public Utilities General Disclosures [Line Items]
O&M expense and regulatory liability			$ 19
Hurricane Irma [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs				$ 103
Hurricane Irma [Member] | Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs				92
Hurricane Irma [Member] | O&M Expenses [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs				4
Hurricane Irma [Member] | Capital Expenditures [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs				$ 7
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
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 revenue requirements	$ 24
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			$ 46		$ 47