Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Regulatory assets:
Regulatory assets	$ 484	$ 458
Less: Current portion	85	88
Long-term regulatory assets	399	370
Regulatory liabilities:
Regulatory liabilities	1,324	1,310
Less: Current portion	82	44
Long-term regulatory liabilities	1,242	1,266
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	70	56
Cost-Recovery Clauses [Member]
Regulatory assets:
Regulatory assets	65	55
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	24	23
Postretirement Benefits [Member]
Regulatory assets:
Regulatory assets	291	295
Storm Reserve [Member]
Regulatory assets:
Regulatory assets	3	3
Other [Member]
Regulatory assets:
Regulatory assets	31	26
Regulatory Tax Liability [Member] | Non-Current Liabilities [Member]
Regulatory liabilities:
Regulatory liabilities	729	715
Cost-Recovery Clauses [Member]
Regulatory liabilities:
Regulatory liabilities	18	17
Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	56	56
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	513	513
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 8	$ 9