Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Regulatory assets:
Regulatory assets	$ 433		$ 421
Less: Current portion	77		28
Long-term regulatory assets	356		393
Regulatory liabilities:
Regulatory liabilities	1,285		745
Less: Current portion	58		154
Long-term regulatory liabilities	1,227		591
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	45		86
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory assets:
Regulatory assets	12		8
Cost-Recovery Clauses - Offsets to Derivative Liabilities [Member]
Regulatory assets:
Regulatory assets	1		0
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	33		37
Postretirement Benefit [Member]
Regulatory assets:
Regulatory assets	272		272
Storm Reserve [Member]
Regulatory assets:
Regulatory assets	47	$ 47	0
Other [Member]
Regulatory assets:
Regulatory assets	23		18
Regulatory Tax Liability [Member] | Non-Current Liabilities [Member]
Regulatory liabilities:
Regulatory liabilities	730		6
Cost-Recovery Clauses - Offsets to Derivative Assets [Member]
Regulatory liabilities:
Regulatory liabilities	0		17
Cost-Recovery Clauses [Member]
Regulatory liabilities:
Regulatory liabilities	32		112
Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	0		56
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	518		547
Other [Member]
Regulatory liabilities:
Regulatory liabilities	$ 5		$ 7