Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Pension Plan [Member]
Components of net periodic benefit cost
Service cost	$ 276	$ 353	$ 552	$ 706
Interest cost	473	370	946	740
Expected return on plan assets	(819)	(954)	(1,638)	(1,908)
Recognized prior service cost		(25)		(50)
Recognized net losses	229	227	458	454
Net periodic cost (benefit)	159	(29)	318	(58)
Supplemental Insurance/ Retirement Plan [Member]
Components of net periodic benefit cost
Service cost	256	277	512	554
Interest cost	482	346	964	693
Recognized prior service cost	28	29	56	58
Recognized net losses	109	176	218	352
Net periodic cost (benefit)	$ 875	$ 828	$ 1,750	$ 1,657