Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Regulatory assets:
Regulatory assets	$ 476	$ 458
Less: Current portion	88	88
Long-term regulatory assets	388	370
Regulatory liabilities:
Regulatory liabilities	1,332	1,310
Less: Current portion	80	44
Long-term regulatory liabilities	1,252	1,266
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	73	56
Cost-Recovery Clauses [Member]
Regulatory assets:
Regulatory assets	61	55
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	25	23
Postretirement Benefits [Member]
Regulatory assets:
Regulatory assets	288	295
Storm Reserve [Member]
Regulatory assets:
Regulatory assets	3	3
Other [Member]
Regulatory assets:
Regulatory assets	26	26
Regulatory Tax Liability [Member] | Non-Current Liabilities [Member]
Regulatory liabilities:
Regulatory liabilities	725	715
Cost-Recovery Clauses [Member]
Regulatory liabilities:
Regulatory liabilities	24	17
Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	56	56
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	512	513
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 15	$ 9