Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Regulatory assets:
Regulatory assets	$ 485	$ 437
Less: Current portion	79	41
Long-term regulatory assets	406	396
Regulatory liabilities:
Regulatory liabilities	1,261	1,303
Less: Current portion	67	93
Long-term regulatory liabilities	1,194	1,210
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	90	74
Cost-Recovery Clauses [Member]
Regulatory assets:
Regulatory assets	38	12
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	22	20
Postretirement Benefits [Member]
Regulatory assets:
Regulatory assets	309	295
Asset Retirement Obligation [Member]
Regulatory assets:
Regulatory assets	13	25
Other [Member]
Regulatory assets:
Regulatory assets	13	11
Regulatory Tax Liability [Member] | Non-Current Liabilities [Member]
Regulatory liabilities:
Regulatory liabilities	691	699
Cost-Recovery Clauses [Member]
Regulatory liabilities:
Regulatory liabilities	23	37
Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	48	48
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	498	506
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 1	$ 13