Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Regulatory assets:
Regulatory assets	$ 419	$ 421
Less: Current portion	29	28
Long-term regulatory assets	390	393
Regulatory liabilities:
Regulatory liabilities	708	745
Less: Current portion	124	154
Long-term regulatory liabilities	584	591
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	85	86
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory assets:
Regulatory assets	10	8
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	35	37
Postretirement Benefits [Member]
Regulatory assets:
Regulatory assets	269	272
Other [Member]
Regulatory assets:
Regulatory assets	20	18
Regulatory Tax Liability [Member]
Regulatory liabilities:
Regulatory liabilities	12	6
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory liabilities:
Regulatory liabilities	91	112
Cost-Recovery Clauses - Offsets to Derivative Assets [Member]
Regulatory liabilities:
Regulatory liabilities	6	17
Transmission and Delivery Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	47	56
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	545	547
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 7	$ 7