Employee Benefits - Net Postretirement Benefit Cost for All Funded and Unfunded Plans (Detail) - Other Postretirement Benefit Plans [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service Cost			$ 0	$ 1	$ 0	$ 1
Interest cost on APBO	$ 1	$ 0	1	2
Expected return on plan assets	(1)	0	(1)	(2)
Amortization of unrecognized prior service credit			0	(1)	$ 0	$ (1)
Net postretirement benefit cost	$ 0	$ 0	$ 0	$ 0