Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Regulatory assets:
Regulatory assets	$ 435	$ 421
Less: Current portion	38	28
Long-term regulatory assets	397	393
Regulatory liabilities:
Regulatory liabilities	581	745
Less: Current portion	65	154
Long-term regulatory liabilities	516	591
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	83	86
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory assets:
Regulatory assets	7	8
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	33	37
Postretirement Benefits [Member]
Regulatory assets:
Regulatory assets	276	272
Storm Reserve [Member]
Regulatory assets:
Regulatory assets	14	0
Other [Member]
Regulatory assets:
Regulatory assets	22	18
Regulatory Tax Liability [Member]
Regulatory liabilities:
Regulatory liabilities	12	6
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory liabilities:
Regulatory liabilities	38	112
Cost-Recovery Clauses - Offsets to Derivative Assets [Member]
Regulatory liabilities:
Regulatory liabilities	0	17
Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	0	56
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	524	547
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 7	$ 7