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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure
Service cost	$ 2	$ 2	$ 7	$ 6
Interest cost	3	4	9	10
Expected return on plan assets	(5)	(5)	(16)	(15)
Amortization of actuarial loss and prior service cost	0	0	2	1
Other	0	0	0	1
Total non-service benefit	(2)	(1)	(5)	(3)
Net periodic benefit cost	$ 0	$ 1	$ 2	$ 3