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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Components of net periodic expense
Interest cost	$ 4	$ 5	$ 8	$ 10
Pension Plans
Components of net periodic expense
Service cost	5	3	10	8
Interest cost	3	5	7	9
Expected return on plan assets	(6)	(5)	(13)	(10)
Amortization of net actuarial loss from earlier periods	3	1	5	3
Loss on settlement of pension obligation	7	0	7	0
Net periodic expense	12	4	16	10
Other Retirement Plans
Components of net periodic expense
Service cost	1	1	2	2
Interest cost	1	0	1	0
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss from earlier periods	0	0	0	0
Loss on settlement of pension obligation	0	0	0	0
Net periodic expense	$ 2	$ 1	$ 3	$ 2