Regulatory - Additional Information (Detail)				1 Months Ended		3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 27, 2017 USD ($) $ / kWac MW	Dec. 15, 2016 USD ($)	Jun. 28, 2016	Sep. 30, 2017 USD ($)	Nov. 30, 2016	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2013	Oct. 31, 2013 USD ($)	Sep. 05, 2013 USD ($)
Public Utilities General Disclosures [Line Items]
Percentage of ROE								10.25%
Return on equity range								Range of plus or minus 1%
Potential increase in ROE								10.50%
Allowed equity in the capital structure										54.00%
Storm costs incurred				$ 10,000,000		$ 10,000,000		$ 10,000,000
Storm costs applied to storm reserve							$ 10,000,000
Storm damage reserve							$ 46,000,000				$ 56,000,000
Regulatory assets				435,000,000		435,000,000		$ 435,000,000	$ 421,000,000
PGS and OPC [Member]
Public Utilities General Disclosures [Line Items]
Settlement agreement, approval date								Feb. 07, 2017
Regulatory assets				$ 32,000,000		32,000,000		$ 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						1,000,000		$ 4,000,000	16,000,000
Regulatory asset amortization beginning period								2016
Regulatory asset amortization ending period								2020
PGS and OPC [Member] | Maximum [Member]
Public Utilities General Disclosures [Line Items]
Decrease bottom return on equity			9.75%
PGS and OPC [Member] | Minimum [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
PGS [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE			10.75%
PGS [Member] | Maximum [Member]
Public Utilities General Disclosures [Line Items]
Return on equity			11.75%
Hurricane Irma [Member]
Public Utilities General Disclosures [Line Items]
Incremental storm restoration costs incurred						70,000,000
Hurricane Irma [Member] | Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Incremental storm restoration costs incurred						60,000,000
Hurricane Irma [Member] | O&M Expenses [Member]
Public Utilities General Disclosures [Line Items]
Incremental storm restoration costs incurred						4,000,000
Hurricane Irma [Member] | Capital Expenditures [Member]
Public Utilities General Disclosures [Line Items]
Incremental storm restoration costs incurred						6,000,000
Natural Gas [Member]
Public Utilities General Disclosures [Line Items]
Financial hedging moratorium period				5 years	1 year
Solar Project Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE	10.25%
Return on equity range	range of plus or minus 1%
Potential increase in ROE	10.50%
Allowed equity in the capital structure	54.00%
Settlement agreement, extended terms	four years through 2021
Settlement agreement, approval date	Nov. 06, 2017
Cost cap of project | $ / kWac	1,500
Cost savings retention percentage for projects below cost cap	25.00%
Solar generation capacity investments	$ 850,000,000
Solar project investment term	4 years
Investments in gas reserves	$ 0
Solar Project Cost Recovery [Member] | Natural Gas [Member]
Public Utilities General Disclosures [Line Items]
Financial hedging moratorium period	5 years
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	150
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	250
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]
Annual accrual, storm damage reserve												$ 8,000,000
Minimum cost recovery period								12 months
Replenishment reserve for recovery of cost											$ 56,000,000
Self-insured storm reserve									56,000,000
Recovery cost in excess of reserve and replenishment of self-insured storm reserve				$ 70,000,000		70,000,000		$ 70,000,000
Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Regulatory assets				14,000,000		14,000,000		14,000,000	$ 0
November 1, 2013 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge				58,000,000		58,000,000		58,000,000
November 1, 2014 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge				8,000,000		8,000,000		8,000,000
November 1, 2015 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge				5,000,000		5,000,000		5,000,000
January 16, 2017 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge				$ 110,000,000		$ 110,000,000		$ 110,000,000
Condition One [Member]
Public Utilities General Disclosures [Line Items]
ROE upper range limit								11.25%
ROE lower range limit								9.25%
Condition One [Member] | Solar Project Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
ROE upper range limit	11.25%
ROE lower range limit	9.25%
Condition Two [Member]
Public Utilities General Disclosures [Line Items]
ROE upper range limit								11.50%
ROE lower range limit								9.50%
Condition Two [Member] | Solar Project Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
ROE upper range limit	11.50%
ROE lower range limit	9.50%
Computer Software [Member]
Public Utilities General Disclosures [Line Items]
Amortization period of Regulatory Asset								15 years