Employee Benefits - Components of Net Periodic Benefit Cost and Changes in Plan Assets and Benefit Obligations Recognized in OCI (Detail) - Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 755	$ 718	$ 852
Interest cost on projected benefit obligation	2,460	2,710	3,326
Net amortization of deferrals	(5,497)	(4,913)	(5,088)
Net periodic benefit cost	(2,282)	(1,485)	$ (910)
Net actuarial loss (gain)	(6,677)	2,895
Recognized actuarial loss	(1,506)	(1,421)
Prior service credit	14,862	0
Recognized prior service credit	7,003	6,334
Total recognized in OCI (before tax effects)	$ (16,042)	$ 7,808