Employee Retirement Benefits - Components of Net Periodic Costs for Qualified and Supplemental Pension Plans (Detail) - Qualified Pension Plans - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure
Service cost	$ 2	$ 2
Interest cost	2	3
Expected return on plan assets	(5)	(5)
Amortization of actuarial loss and prior service cost	2	1
Other	2	0
Total non-service cost (benefit)	1	(1)
Net periodic benefit cost	$ 3	$ 1