Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 57	$ 36	$ 175	$ 107
Interest cost	27	18	82	54
Expected return on plan assets	(31)	(25)	(94)	(75)
Amortization – prior service credit	(11)	(9)	(34)	(25)
Amortization – actuarial loss	5	9	16	26
Amortization - settlement loss	7	45	9	47
Net periodic benefit cost	54	74	154	134
Other Benefits
Components of net periodic benefit cost:
Service cost	8	7	24	22
Interest cost	9	7	28	22
Expected return on plan assets	0	0	0	0
Amortization – prior service credit	0	0	0	(2)
Amortization – actuarial loss	0	(1)	(1)	(1)
Amortization - settlement loss	0	0	0	0
Net periodic benefit cost	$ 17	$ 13	$ 51	$ 41