Pension and Other Postretirement Benefits (Components of Net Periodic Benefit Costs) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 83	$ 60	$ 152	$ 118
Interest cost	24	27	49	55
Expected return on plan assets	(32)	(31)	(66)	(63)
Amortization – prior service credit	(12)	(12)	(23)	(23)
Amortization – actuarial loss	10	7	18	11
Amortization - settlement loss	1	2	1	2
Net periodic benefit cost	74	53	131	100
Other Benefits
Components of net periodic benefit cost:
Service cost	9	8	18	16
Interest cost	9	10	17	19
Expected return on plan assets	0	0	0	0
Amortization – prior service credit	0	0	0	0
Amortization – actuarial loss	0	(1)	1	(1)
Amortization - settlement loss	0	0	0	0
Net periodic benefit cost	$ 18	$ 17	$ 36	$ 34