Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Regulatory assets:
Regulatory assets	$ 426	$ 421
Less: Current portion	26	28
Long-term regulatory assets	400	393
Regulatory liabilities:
Regulatory liabilities	662	745
Less: Current portion	87	154
Long-term regulatory liabilities	575	591
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	84	86
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory assets:
Regulatory assets	8	8
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	34	37
Postretirement Benefits [Member]
Regulatory assets:
Regulatory assets	279	272
Other [Member]
Regulatory assets:
Regulatory assets	21	18
Regulatory Tax Liability [Member]
Regulatory liabilities:
Regulatory liabilities	12	6
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory liabilities:
Regulatory liabilities	59	112
Cost-Recovery Clauses - Offsets to Derivative Assets [Member]
Regulatory liabilities:
Regulatory liabilities	1	17
Transmission and Delivery Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	47	56
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	536	547
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 7	$ 7