Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory assets:
Regulatory assets	$ 458	$ 433
Less: Current portion	88	77
Long-term regulatory assets	370	356
Regulatory liabilities:
Regulatory liabilities	1,310	1,285
Less: Current portion	44	58
Long-term regulatory liabilities	1,266	1,227
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	56	45
Cost-Recovery Clauses [Member]
Regulatory assets:
Regulatory assets	55	13
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	23	33
Postretirement Benefit [Member]
Regulatory assets:
Regulatory assets	295	272
Storm Reserve [Member]
Regulatory assets:
Regulatory assets	3	47
Regulatory liabilities:
Regulatory liabilities	56
Other [Member]
Regulatory assets:
Regulatory assets	26	23
Regulatory Tax Liability [Member] | Non-Current Liabilities [Member]
Regulatory liabilities:
Regulatory liabilities	715	730
Cost-Recovery Clauses [Member]
Regulatory liabilities:
Regulatory liabilities	17	32
Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	56	0
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	513	518
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 9	$ 5