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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Components of net periodic expense
Interest cost	$ 4	$ 4	$ 12	$ 14
Pension Plans
Components of net periodic expense
Service cost	4	5	14	13
Interest cost	4	4	11	13
Expected return on plan assets	(7)	(5)	(20)	(15)
Amortization of net actuarial loss from earlier periods	3	2	8	5
Loss on settlement of pension obligation	1	0	8	0
Net periodic expense	5	6	21	16
Other Retirement Plans
Components of net periodic expense
Service cost	1	0	3	2
Interest cost	0	1	1	1
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	$ 1	$ 1	$ 4	$ 3