Pension and Postretirement Benefits Other Than Pension Plans - Net Postretirement Benefit Income (Detail) - Other Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 24	$ 28	$ 48	$ 54
Amortization of net actuarial loss	65	66	129	123
Amortization of prior service credit	(329)	(329)	(657)	(657)
Total net periodic pension (income) expense	$ (240)	$ (235)	$ (480)	$ (480)