Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Regulatory assets:
Regulatory assets	$ 392	$ 433
Less: Current portion	45	77
Long-term regulatory assets	347	356
Regulatory liabilities:
Regulatory liabilities	1,266	1,285
Less: Current portion	61	58
Long-term regulatory liabilities	1,205	1,227
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	44	45
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory assets:
Regulatory assets	22	13
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	29	33
Postretirement Benefits [Member]
Regulatory assets:
Regulatory assets	263	272
Storm Reserve [Member]
Regulatory assets:
Regulatory assets	9	47
Other [Member]
Regulatory assets:
Regulatory assets	25	23
Regulatory Tax Liability [Member] | Non-Current Liabilities [Member]
Regulatory liabilities:
Regulatory liabilities	719	730
Tax Reform and Storm Agreement [Member]
Regulatory liabilities:
Regulatory liabilities	13	0
Cost Recovery Clauses [Member]
Regulatory liabilities:
Regulatory liabilities	17	32
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	512	518
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 5	$ 5