Regulatory - Additional Information (Detail) - USD ($)		1 Months Ended	12 Months Ended
	Jun. 28, 2016	May 31, 2009	Dec. 31, 2016	Dec. 31, 2013	Dec. 31, 2015
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%
Amortization period of Regulatory Asset			12 years
Allowed equity in the capital structure		54.70%		54.00%
Annual accrual storm damage reserve				$ 8,000,000
Storm damage reserve			$ 56,100,000		$ 56,100,000
Storm damage cost recovery period			12-month period or longer
Storm costs incurred			$ 8,600,000
Regulatory assets			420,700,000		$ 418,100,000
Minimum [Member]
Public Utilities General Disclosures [Line Items]
Return on equity		9.75%
Maximum [Member]
Public Utilities General Disclosures [Line Items]
Return on equity		11.75%
PGS [Member]
Public Utilities General Disclosures [Line Items]
Percentage of ROE		10.75%
Reduction in annual depreciation expense			$ 16,100,000
Settlement agreement date			Apr. 18, 2016
PGS and OPC [Member]
Public Utilities General Disclosures [Line Items]
Reduction in annual depreciation expense			$ 16,100,000
Decrease bottom return on equity	9.25%
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			$ 16,000,000
PGS and OPC [Member] | Minimum [Member]
Public Utilities General Disclosures [Line Items]
Amortization period of Regulatory Asset			2 years
Litigation settlement, amount			$ 21,000,000
November 1, 2013 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge			57,500,000
November 1, 2014 [Member]
Public Utilities General Disclosures [Line Items]
Additional Revenue generated from increase in service charge			7,500,000
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