Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Pension Plan [Member]
Components of net periodic benefit cost
Service cost	$ 276	$ 353	$ 828	$ 1,059
Interest cost	473	371	1,419	1,111
Expected return on plan assets	(819)	(954)	(2,457)	(2,863)
Recognized prior service cost		(25)		(75)
Recognized net losses	229	226	687	680
Net periodic benefit cost (credit)	159	(29)	477	(88)
Supplemental Insurance/ Retirement Plan [Member]
Components of net periodic benefit cost
Service cost	256	277	768	831
Interest cost	482	346	1,445	1,040
Recognized prior service cost	28	29	86	87
Recognized net losses	109	176	326	527
Net periodic benefit cost (credit)	$ 875	$ 828	$ 2,625	$ 2,485