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Restructuring and Severance Related Expenses - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Restructuring Reserve [Roll Forward]
Beginning Balance			$ 5
Restructuring and severance related expenses			2
Payments			(3)
Ending Balance	$ 4		4
Operations and Maintenance
Restructuring Reserve [Roll Forward]
Restructuring and severance related expenses	0	$ 1	2	$ 3
Depreciation And Amortization
Restructuring Reserve [Roll Forward]
Restructuring and severance related expenses	$ 1	$ 1	$ 3	$ 1