Employee Benefit Plans and Equity Compensation Plan (Schedule of Amounts in Regulatory Assets to be Recognized Over the Next Fiscal Year) (Detail) - Scenario Forecast $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, future amortization of gain or loss from regulatory assets	$ 6
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, future amortization of gain or loss from regulatory assets	$ 1