Regulatory - Additional Information (Detail) - USD ($)			3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 15, 2016	Jun. 28, 2016	Jun. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2013
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			$ 9,000,000	$ 9,000,000
Regulatory assets			426,000,000	$ 426,000,000	$ 421,000,000
PGS and OPC [Member]
Public Utilities General Disclosures [Line Items]
Reduction in annual depreciation expense	$ 16,000,000
Date new bottom of return on equity range will remain in effect				Dec. 31, 2020
Settlement agreement date				February 7, 2017
Regulatory assets			32,000,000	$ 32,000,000
Amortization expenses			2,000,000	$ 3,000,000	$ 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%
November 1, 2013 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge			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
November 1, 2015 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge			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
Condition One [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%
Computer Software [Member]
Public Utilities General Disclosures [Line Items]
Amortization period of Regulatory Asset				15 years