Regulatory Matters - Other Regulatory Matters - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Public Utilities, General Disclosures [Line Items]
Annual electric revenue subject to potential refund		$ 249,000,000
Revenues from gas		32,000,000
Annual steam revenue subject to potential refund		6,000,000
Potential estimated refund to customers		1,962,000,000
Construction expenditures overcharge consultant estimate	$ 208,000,000
Regulatory liability, expected credit to customers		116,000,000
Regulatory liability, return on capital expenditures, not recoverable		55,000,000
Regulatory liabilities		$ 1,977,000,000	$ 1,993,000,000
Net electric deferrals
Public Utilities, General Disclosures [Line Items]
Amortization period (years)		10 years
Prudence proceeding
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities		$ 99,000,000	105,000,000
Net unbilled revenue deferrals
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities		$ 109,000,000	$ 138,000,000