Regulatory - Additional Information (Detail) - USD ($)			3 Months Ended	12 Months Ended
	Dec. 15, 2016	Jun. 28, 2016	Mar. 31, 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
Regulatory assets			$ 419,000,000	$ 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
Regulatory asset amortization beginning period			2016
Regulatory asset amortization ending period			2020
Amortization expenses			$ 1,000,000	$ 16,000,000
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%
Litigation settlement, amount			$ 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
November 1, 2014 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge			8,000,000
November 1, 2015 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge			5,000,000
January 16, 2017 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge			$ 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