Employee Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Change in plan assets
Balance at end of year	$ 33,812	$ 32,322
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Total recognized in other comprehensive income	15,369	7,051
Defined Benefit Pension Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	29,879	31,910
Service cost	1,034	1,196
Interest cost	1,467	1,257
Actuarial (gain)/loss	8,398	(3,734)
Benefits paid	(767)	(750)
Projected benefit obligation at end of year	40,011	29,879
Change in plan assets
Balance at beginning of year	32,322	23,983
Actual return on plan assets	1,337	4,619
Employer contributions	920	4,470
Benefits paid	(767)	(750)
Balance at end of year	33,812	32,322
(Unfunded) Funded status	(6,199)	2,443
Accumulated benefit obligation	40,011	29,747
Weighted-average assumptions as of December 31
Discount rate - Liability	4.00%	5.00%
Discount rate - Expense	5.00%	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	1,034	1,196
Interest cost	1,467	1,257
Expected return on plan assets	(2,543)	(1,880)
Recognized prior service cost	(104)	(104)
Recognized net losses	12	630
Net periodic cost	(134)	1,099
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost	104	104
Net (gain) loss	9,592	(6,956)
Total recognized in other comprehensive income	9,696	(6,852)
Total recognized in net periodic benefit cost and other comprehensive income	9,562	(5,753)
Supplemental Insurance/ Retirement Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	25,502	25,835
Service cost	1,555	1,554
Interest cost	1,325	1,072
Actuarial (gain)/loss	4,650	(1,916)
Benefits paid	(1,043)	(1,043)
Projected benefit obligation at end of year	31,989	25,502
Change in plan assets
Benefits paid	(1,043)	(1,043)
(Unfunded) Funded status	(31,989)	(25,502)
Accumulated benefit obligation	29,023	22,278
Weighted-average assumptions as of December 31
Discount rate - Liability	4.00%	5.00%
Discount rate - Expense	5.00%	4.00%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	1,555	1,554
Interest cost	1,325	1,072
Recognized prior service cost	114	114
Recognized net losses	355	514
Net periodic cost	3,349	3,254
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost	(114)	(114)
Net (gain) loss	4,268	(2,401)
Total recognized in other comprehensive income	4,154	(2,515)
Total recognized in net periodic benefit cost and other comprehensive income	$ 7,503	$ 739