Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Regulatory assets:
Regulatory assets	$ 386	$ 433
Less: Current portion	46	77
Long-term regulatory assets	340	356
Regulatory liabilities:
Regulatory liabilities	1,298	1,285
Less: Current portion	96	58
Long-term regulatory liabilities	1,202	1,227
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	45	45
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory assets:
Regulatory assets	36	13
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	23	33
Postretirement Benefits [Member]
Regulatory assets:
Regulatory assets	259	272
Storm Reserve [Member]
Regulatory assets:
Regulatory assets	0	47
Regulatory liabilities:
Regulatory liabilities	56
Other [Member]
Regulatory assets:
Regulatory assets	23	23
Regulatory Tax Liability [Member] | Non-Current Liabilities [Member]
Regulatory liabilities:
Regulatory liabilities	722	730
Tax Reform and Storm Agreement [Member]
Regulatory liabilities:
Regulatory liabilities	11	0
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory liabilities:
Regulatory liabilities	25	32
Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	25	0
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	511	518
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 4	$ 5