Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory assets:
Regulatory assets	$ 437	$ 458
Less: Current portion	41	88
Long-term regulatory assets	396	370
Regulatory liabilities:
Regulatory liabilities	1,303	1,310
Less: Current portion	93	44
Long-term regulatory liabilities	1,210	1,266
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	74	56
Cost-Recovery Clauses [Member]
Regulatory assets:
Regulatory assets	12	55
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	20	23
Postretirement Benefits [Member]
Regulatory assets:
Regulatory assets	295	295
Asset Retirement Obligation [Member]
Regulatory assets:
Regulatory assets	25	18
Other [Member]
Regulatory assets:
Regulatory assets	11	11
Regulatory Tax Liability [Member] | Non-Current Liabilities [Member]
Regulatory liabilities:
Regulatory liabilities	699	715
Cost-Recovery Clauses [Member]
Regulatory liabilities:
Regulatory liabilities	37	17
Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	48	56
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	506	513
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 13	$ 9