Defined Benefit Pension Plan and Postretirement Benefits - Net Periodic Benefit Cost for Pension Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost	$ 100	$ 109	$ 198	$ 223
Interest cost	62	48	124	98
Recognized actuarial (gains)	(21)	(14)	(42)	(29)
Net periodic benefit cost	$ 141	$ 143	$ 280	$ 292