Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) - Defined Contribution and Other Retirement Plans [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 221	$ 214	$ 442	$ 429
Interest cost on projected benefit obligation	588	787	1,176	1,574
Net amortization and deferrals	(1,046)	(1,355)	(2,092)	(2,710)
Total postretirement benefit income	$ (237)	$ (354)	$ (474)	$ (707)