Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1		$ 2
Interest cost	7	$ 6	14	$ 12
Expected return on assets	(9)	(10)	(18)	(20)
Recognized actuarial (gain) loss	3	4	7	8
Net periodic benefit cost (credit)	2		5
Other Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost			1	1
Recognized actuarial (gain) loss	(2)	(2)	(4)	(4)
Amortization of prior service credit			(1)	(1)
Net periodic benefit cost (credit)	$ (2)	$ (2)	$ (4)	$ (4)