Employee Retirement Benefits - Components of Net Periodic Costs for Qualified and Supplemental Pension Plans (Detail) - Qualified Pension Plans - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure
Service cost	$ 2	$ 2	$ 7	$ 7
Interest cost	2	3	7	9
Expected return on plan assets	(5)	(5)	(16)	(16)
Amortization of actuarial loss and prior service cost	2	0	5	2
Other	0	0	2	0
Total non-service benefit	(1)	(2)	(2)	(5)
Net periodic benefit cost	$ 1	$ 0	$ 5	$ 2