Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 60	$ 35	$ 118	$ 71
Interest cost	27	18	55	36
Expected return on plan assets	(31)	(24)	(63)	(50)
Amortization – prior service credit	(12)	(8)	(23)	(16)
Amortization – actuarial loss	7	8	11	17
Amortization - settlement loss	2	1	2	2
Net periodic benefit cost	53	30	100	60
Other Benefits
Components of net periodic benefit cost:
Service cost	8	8	16	15
Interest cost	10	8	19	15
Expected return on plan assets	0	0	0	0
Amortization – prior service credit	0	(1)	0	(2)
Amortization – actuarial loss	(1)	0	(1)	0
Amortization - settlement loss	0	0	0	0
Net periodic benefit cost	$ 17	$ 15	$ 34	$ 28