Post-Retirement and Similar Obligations - Periodic Benefit Costs Net (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12	$ 10	$ 35	$ 30
Interest cost	27	33	81	98
Expected return on plan assets	(50)	(48)	(150)	(144)
Prior service costs	0	0	0	(1)
Actuarial loss	31	28	94	85
Net Periodic Benefit Cost	20	23	60	68
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	2	2
Interest cost	4	4	10	12
Expected return on plan assets	(2)	(1)	(6)	(5)
Prior service costs	(3)	(3)	(7)	(7)
Actuarial loss	0	0	1	(1)
Net Periodic Benefit Cost	$ 0	$ 1	$ 0	$ 1