Summary of Composition of Regulatory Assets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Asset [Line Items]
Regulatory assets	$ 1,289	$ 1,271
Deferred Pension Expense
Regulatory Asset [Line Items]
Regulatory assets	310	269
Income Taxes Recoverable through Rates
Regulatory Asset [Line Items]
Regulatory assets	241	236
Removal Costs Recoverable through Rates
Regulatory Asset [Line Items]
Regulatory assets	251	225
Deferred Other Postretirement Benefit Expense
Regulatory Asset [Line Items]
Regulatory assets	14	90
San Clemente Dam Project Costs
Regulatory Asset [Line Items]
Regulatory assets	91	95
Regulatory Balancing Accounts
Regulatory Asset [Line Items]
Regulatory assets	110	88
Debt Expense
Regulatory Asset [Line Items]
Regulatory assets	66	68
Purchase Premium Recoverable through Rates
Regulatory Asset [Line Items]
Regulatory assets	58	60
Deferred Tank Painting Costs
Regulatory Asset [Line Items]
Regulatory assets	39	37
Other
Regulatory Asset [Line Items]
Regulatory assets	$ 109	$ 103