Pension and Employee Benefit Plans - Schedule of Changes in Projected Benefit Obligations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in projected benefit obligation:
Service cost	$ 415	$ 447	$ 233
Interest cost	1,314	1,426	1,412
Change in plan assets:
Fair value of plan assets at beginning of year	24,063
Fair value of plan assets at end of year	26,836	24,063
Accumulated other comprehensive income:
Amount recognized in accumulated other comprehensive income	$ (5,161)	$ (5,083)	$ (5,170)	$ (4,878)
Weighted-average assumptions used to determine benefit obligations for the year ended December 31:
Discount rate	3.85%	4.00%	3.75%
Compensation increase rate	0.00%	0.00%	0.00%
Pension Benefits [Member]
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 35,012	$ 36,140	$ 38,208
Service cost	415	447	233
Interest cost	1,314	1,426	1,412
Actuarial loss/(gain)	1,787	301	(1,744)
Administrative expenses	(402)	(606)	(214)
Benefits paid	(1,799)	(2,696)	(1,755)
Projected benefit obligation at end of year	36,327	35,012	36,140
Change in plan assets:
Fair value of plan assets at beginning of year	24,063	25,296	27,302
Actual return (loss) on plan assets	3,152	2,040	(443)
Employer contribution	1,822	29	406
Administrative expenses	(402)	(606)	(214)
Benefits paid	(1,799)	(2,696)	(1,755)
Fair value of plan assets at end of year	26,836	24,063	25,296
Unfunded status	(9,491)	(10,949)	(10,844)
Defined benefit liabilities included in other liabilities	(9,491)	(10,949)	(10,844)
Deferred tax benefit associated with accumulated other comprehensive loss	3,153	3,107	3,154
Accumulated other comprehensive loss, net of tax	5,154	5,074	5,144
Net amount recognized	(1,184)	(2,768)	(2,546)
Other comprehensive income (loss):
Net loss (gain)	358	90	708
Amortization of net actuarial (gain)/loss	(227)	(212)	(258)
Total recognized in other comprehensive income (loss)	131	(122)	450
Accumulated other comprehensive income:
Net loss (gain)	8,307	8,181	8,298
Amount recognized in accumulated other comprehensive income	$ 8,307	$ 8,181	$ 8,298
Weighted-average assumptions used to determine benefit obligations for the year ended December 31:
Discount rate	3.35%	3.85%	4.00%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Change in projected benefit obligation:
Accumulated postretirement benefit obligation at beginning of year	$ 131	$ 159	$ 155
Interest cost	4	4	5
Amendments			9
Actuarial loss/(gain)	3	(8)	18
Benefits paid	(26)	(24)	(28)
Accumulated postretirement benefit obligation at end of year	112	131	159
Change in plan assets:
Employer contribution	26	24	28
Benefits paid	(26)	(24)	(28)
Unfunded status	(112)	(131)	(159)
Defined benefit liabilities included in accounts payable and accrued expenses	(24)	(25)	(26)
Defined benefit liabilities included in other liabilities	(88)	(106)	(133)
Deferred tax benefit associated with accumulated other comprehensive loss	12	15	15
Accumulated other comprehensive loss, net of tax	7	9	27
Net amount recognized	(93)	(107)	(117)
Other comprehensive income (loss):
Net loss (gain)	3	(9)	17
Prior service cost			9
Amortization of prior service cost	(11)	(10)	(9)
Amortization of net actuarial (gain)/loss	3	1	3
Total recognized in other comprehensive income (loss)	(5)	(18)	20
Accumulated other comprehensive income:
Net loss (gain)	(22)	(28)	(20)
Prior service cost	41	52	62
Amount recognized in accumulated other comprehensive income	$ 19	$ 24	$ 42
Weighted-average assumptions used to determine benefit obligations for the year ended December 31:
Discount rate	2.65%	2.75%	3.00%