Pension Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost Table (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits [Member]
Components of net periodic benefit cost
Service cost	$ 22	$ 23
Interest cost	20	19
Expected (return) loss on plan assets	(21)	(21)
Amortization of net actuarial loss (gain)	4	7
Amortization of net prior service cost (credit)	0	0
Settlement expense	4	0
Net periodic benefit cost	29	28
Other Benefits [Member]
Components of net periodic benefit cost
Service cost	0	0
Interest cost	3	3
Expected (return) loss on plan assets	0	0
Amortization of net actuarial loss (gain)	0	0
Amortization of net prior service cost (credit)	(1)	(6)
Settlement expense	0	0
Net periodic benefit cost	$ 2	$ (3)