Employee Retirement Benefits - Components of Net Periodic Costs for Qualified and Supplemental Pension Plans (Detail) - Qualified Pension Plans - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan Disclosure
Service cost	$ 3	$ 3	$ 5	$ 5
Interest cost	3	3	5	6
Expected return on plan assets	(6)	(6)	(11)	(11)
Amortization of actuarial loss and prior service cost	1	1	3	2
Other	0	0	2	0
Total non-service benefit	(2)	(2)	(1)	(3)
Net periodic benefit cost	$ 1	$ 1	$ 4	$ 2