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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost	$ 183	$ 108	$ 364	$ 213
Interest cost	61	52	122	103
Recognized actuarial (gains)	(2)	(23)	(3)	(46)
Net periodic benefit cost	$ 242	$ 137	$ 483	$ 270