Regulatory Assets and Liabilities - Current and Non-Current Assets (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Total regulatory assets	$ 2,867	$ 2,861
Less: current portion	299	294
Total non-current regulatory assets	2,568	2,567
Pension and other post-retirement benefits cost deferrals
Regulatory Assets [Line Items]
Total regulatory assets	113	125
Pension and other post-retirement benefits
Regulatory Assets [Line Items]
Total regulatory assets	1,027	1,061
Storm costs
Regulatory Assets [Line Items]
Total regulatory assets	332	272
Rate adjustment mechanism
Regulatory Assets [Line Items]
Total regulatory assets	28	79
Revenue decoupling mechanism
Regulatory Assets [Line Items]
Total regulatory assets	61	19
Transmission revenue reconciliation mechanism
Regulatory Assets [Line Items]
Total regulatory assets	5	5
Contracts for differences
Regulatory Assets [Line Items]
Total regulatory assets	95	92
Hardship programs
Regulatory Assets [Line Items]
Total regulatory assets	24	29
Plant decommissioning
Regulatory Assets [Line Items]
Total regulatory assets	3	5
Deferred purchased gas
Regulatory Assets [Line Items]
Total regulatory assets	2	25
Deferred transmission expense
Regulatory Assets [Line Items]
Total regulatory assets	19	11
Environmental remediation costs
Regulatory Assets [Line Items]
Total regulatory assets	273	277
Debt premium
Regulatory Assets [Line Items]
Total regulatory assets	92	97
Unamortized losses on reacquired debt
Regulatory Assets [Line Items]
Total regulatory assets	29	29
Unfunded future income taxes
Regulatory Assets [Line Items]
Total regulatory assets	402	399
Federal tax depreciation normalization adjustment
Regulatory Assets [Line Items]
Total regulatory assets	152	153
Asset retirement obligation
Regulatory Assets [Line Items]
Total regulatory assets	20	17
Deferred meter replacement costs
Regulatory Assets [Line Items]
Total regulatory assets	26	27
Other
Regulatory Assets [Line Items]
Total regulatory assets	$ 164	$ 139