Pension and Other Retirement Benefits - Components of Net Periodic Benefit Expense Related to Post-Retirement Plans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Components of net periodic expense
Interest cost	$ 4	$ 6	$ 14	$ 17
Pension Plans
Components of net periodic expense
Service cost	5	4	13	12
Interest cost	4	5	13	15
Expected return on plan assets	(5)	(5)	(15)	(15)
Amortization of net actuarial loss from earlier periods	2	1	5	3
Net periodic expense	6	5	16	15
Other Retirement Plans
Components of net periodic expense
Service cost	0	1	2	2
Interest cost	1	0	1	1
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss from earlier periods	0	0	0	0
Net periodic expense	$ 1	$ 1	$ 3	$ 3