Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets
Balance at beginning of year	$ 33,717
Balance at end of year	37,447	$ 33,717
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Total recognized in other comprehensive income	16,733	17,406
Defined Benefit Pension Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	38,597	40,011
Service cost	1,273	1,343
Interest cost	1,358	1,576
Actuarial (gain)/loss	2,593	(3,424)
Benefits paid	(1,566)	(909)
Projected benefit obligation at end of year	42,255	38,597
Change in plan assets
Balance at beginning of year	33,717	33,812
Actual return on plan assets	3,221	(1,186)
Employer contributions	2,075	2,000
Benefits paid	(1,566)	(909)
Balance at end of year	37,447	33,717
(Unfunded) Funded status	(4,808)	(4,880)
Accumulated benefit obligation	$ 42,255	$ 38,597
Weighted-average assumptions as of December 31
Discount rate - Liability	3.99%	4.18%
Discount rate - Expense	4.18%	4.00%
Expected return on plan assets	8.00%	8.00%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	$ 1,273	$ 1,343
Interest cost	1,358	1,576
Expected return on plan assets	(2,776)	(2,749)
Recognized prior service cost	(104)	(104)
Recognized net losses	801	812
Net periodic cost (benefit)	552	878
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost	104	104
Net (gain) loss	1,347	(301)
Total recognized in other comprehensive income	1,451	(197)
Total recognized in net periodic benefit cost and other comprehensive income	2,003	681
Supplemental Insurance/ Retirement Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	38,204	31,989
Service cost	1,820	1,589
Interest cost	1,334	1,365
Actuarial (gain)/loss	(1,653)	4,304
Benefits paid	(1,095)	(1,043)
Projected benefit obligation at end of year	38,610	38,204
Change in plan assets
Benefits paid	(1,095)	(1,043)
(Unfunded) Funded status	(38,610)	(38,204)
Accumulated benefit obligation	$ 36,392	$ 34,884
Weighted-average assumptions as of December 31
Discount rate - Liability	3.85%	4.03%
Discount rate - Expense	4.01%	4.00%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	$ 1,820	$ 1,589
Interest cost	1,334	1,365
Recognized prior service cost	114	114
Recognized net losses	805	599
Net periodic cost (benefit)	4,073	3,667
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost	(114)	(114)
Net (gain) loss	(2,458)	3,705
Total recognized in other comprehensive income	(2,572)	3,591
Total recognized in net periodic benefit cost and other comprehensive income	$ 1,501	$ 7,258