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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure
Service cost	$ 3	$ 2	$ 5	$ 4
Interest cost	3	3	6	6
Expected return on plan assets	(6)	(5)	(11)	(10)
Amortization of actuarial loss and prior service cost	1	1	2	1
Other	0	0	0	1
Total non-service benefit	(2)	(1)	(3)	(2)
Net periodic benefit cost	$ 1	$ 1	$ 2	$ 2