Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 8	$ 8	$ 8
Interest cost	110	119	128
Expected return on plan assets	(179)	(173)	(177)
Amortization of unrecognized net loss	42	43	46
Amortization of unrecognized prior service benefit			(1)
Settlement	9	11	3
Net periodic benefit cost	(10)	8	7
Net periodic benefit cost	(10)	8	7
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	1
Interest cost	2	2	3
Expected return on plan assets	(5)	(5)	(5)
Amortization of unrecognized net loss	(1)
Amortization of unrecognized prior service benefit	(2)	(2)	(3)
Settlement	0	0	0
Net periodic benefit cost	(5)	(4)	(4)
Net periodic benefit cost	$ (5)	$ (4)	$ (4)