Employee Benefits - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Number of consecutive years of employee's compensation	5 years
Number of total years of employee's compensation	10 years
Accumulated benefit obligation	$ 101,600,000	$ 104,300,000
Pension contributions	0
Projected benefit obligation	109,747,000	113,124,000	$ 95,969,000
Net periodic benefit cost	1,736,000	736,000	2,832,000
Level 1 to Level 2 transfers	0	0
Purchases, transfers or sales of level 3 plan assets	0
Scenario, Forecast
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss included in accumulated other comprehensive income (loss)				$ (2,300,000)
Prior service credit expected recognized Amount				$ 100,000
Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	4,000,000	3,500,000
Net periodic benefit cost	600,000	400,000	400,000
Defined benefit plan in accumulated other comprehensive gain/loss before tax	$ 2,100,000
Thrift Plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to the Thrift Plan	3.50%
Compensation expense	$ 1,500,000	$ 1,300,000	$ 1,400,000
Marketable equity securities
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation minimum	50.00%
Targeted asset allocation maximum	70.00%
Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation minimum	30.00%
Targeted asset allocation maximum	50.00%