Regulatory Matters - Additional Information (Detail) - USD ($)	Mar. 31, 2016	Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]
Expected credit to customers	$ 116,000,000
Return on capital expenditures, not recoverable (up to)	55,000,000
Regulatory liabilities	1,858,000,000	$ 1,977,000,000
Prudence proceeding
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	$ 98,000,000	$ 99,000,000