Leases - Summary of Lease Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Assets And Liabilities Lessee [Line Items]
Total lease liabilities	$ 27	$ 29
Other Deferred Debits [Member]
Schedule Of Assets And Liabilities Lessee [Line Items]
Right-of-use asset	26	28
Other Current Liabilities [Member]
Schedule Of Assets And Liabilities Lessee [Line Items]
Current	2	2
Deferred Credits and Other Liabilities [Member]
Schedule Of Assets And Liabilities Lessee [Line Items]
Long-term	$ 25	$ 27