Post-Retirement and Similar Obligations - Periodic Benefit Costs Net (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12	$ 10
Interest cost	27	33
Expected return on plan assets	(50)	(48)
Amortization of actuarial loss	31	30
Net Periodic Benefit Cost	20	25
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1
Interest cost	3	4
Expected return on plan assets	(2)	(2)
Amortization of prior service costs	(2)	(2)
Net Periodic Benefit Cost	$ 0	$ 1