Pension and Other Postretirement Benefits (Components of Net Periodic Benefit Costs) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 73	$ 70	$ 221	$ 210
Interest cost	23	25	69	73
Expected return on plan assets	(37)	(33)	(99)	(98)
Amortization – prior service credit	(11)	(12)	(34)	(34)
Amortization – actuarial loss	10	9	35	26
Amortization - settlement loss	15	8	64	9
Net periodic benefit cost	73	67	256	186
Other Benefits
Components of net periodic benefit cost:
Service cost	9	9	26	27
Interest cost	7	8	22	24
Amortization – prior service credit	1	0	2	0
Amortization – actuarial loss	2	1	7	2
Net periodic benefit cost	$ 19	$ 18	$ 57	$ 53