Pension and Other Retirement Benefits - Components of Net Periodic Benefit Expense Related to Post-Retirement Plans (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Components of net periodic expense
Interest cost	$ 5	$ 5	$ 10	$ 11
Pension Plans
Components of net periodic expense
Service cost	3	4	8	8
Interest cost	5	5	9	10
Expected return on plan assets	(5)	(5)	(10)	(10)
Amortization of net actuarial loss from earlier periods	1	1	3	2
Net periodic expense	4	5	10	10
Other Retirement Plans
Components of net periodic expense
Service cost	1	1	2	1
Interest cost	0	0	0	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	$ 2	$ 2