Employee and Director Benefit Plans - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jan. 01, 1995	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Working Hours service needed for the eligibility of ESOP during a plan year	1000 hours
Eligible age of employee for participation in ESOP	21 years
Employer Matching Contribution Percentage	75.00%
Percentage of compensation contributed by employees	6.00%
Total contributions to the plans charged to operating expense	$ 280,000	$ 220,000	$ 330,000
Employee Stock Ownership Plan (ESOP), Compensation Expense	7,678,640	7,594,790	7,691,059
ESOP held allocated shares	315,269	359,030	383,141
Age limit for receipt of annual directors fees	65 years
Interest on deferred fees accrues	10.00%
Cash surrender value of life insurance	18,145,000	17,456,000	16,861,000
Accrual amounted Projected unit cost included in Employee and director benefit plans liabilities					517,599
Employees service years required for health Insurance	25 years
Continuous years of service for attaining voluntary early retirement program				25 years
Accumulated post-retirement benefit obligation amortization year	20 years
Accumulated post retirement benefit obligation				3,799,308
Defined Benefit Plan Health Care Cost Trend Rate Assumed For Current Fiscal Year	7.00%
Defined Benefit Plan Health Care Cost Trend Rate Assumed For Year Five and After	5.00%
Defined Benefit Plan Increase in Assumed Health care Cost Trend Rate Percentage	1.00%
Defined Benefit Plan Decrease in Assumed Health care Cost Trend Rate Percentage	1.00%
Defined Benefit Plan Effect of One Percentage Point Increase on Accumulated Post retirement Benefit Obligation Percentage	16.45%
Accumulated post-retirement benefit obligation decreased	13.02%
Prior service credit	81,000
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Social Security normal retirement age	65 years
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Social Security normal retirement age	67 years
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit						81,381
Service and Interest Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Effect of One Percentage Point Increase on Accumulated Post retirement Benefit Obligation Percentage	16.78%
Accumulated post-retirement benefit obligation decreased	13.54%
Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash surrender value of life insurance	277,278	269,271	262,466
Present value of accumulated benefits Interest rate	4.50%	4.50%	5.25%
Accrual amounted Projected unit cost included in Employee and director benefit plans liabilities	80,997	78,759	68,253
January 1, 2014 Amendment [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase in accumulated post-retirement benefit obligation		1,150,229
Age of eligibility to participate in retiree health plan	60 years
Net Periodic Post-retirement Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Present value of accumulated benefits Interest rate	4.00%	4.80%	4.00%
Chief Executive Officer [Member]
Defined Benefit Plan Disclosure [Line Items]
Normal retirement benefits percentage	67.00%
Executive Vice President [Member]
Defined Benefit Plan Disclosure [Line Items]
Normal retirement benefits percentage	58.00%
Executive Officer [Member]
Defined Benefit Plan Disclosure [Line Items]
Normal retirement benefits percentage	50.00%
Supplemental retirement benefits period	15 years
Cash surrender value of life insurance	1,346,910	1,218,175	1,105,741
Present value of accumulated benefits Interest rate	4.50%	4.50%	5.25%
Accrual amounted Projected unit cost included in Employee and director benefit plans liabilities	1,450,280	1,435,554	1,328,657
Direct [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash surrender value of life insurance	16,370,384	15,824,497	15,363,241
Present value of accumulated benefits Interest rate	4.50%	4.50%	5.25%
Accrual amounted Projected unit cost included in Employee and director benefit plans liabilities	11,465,119	11,004,738	10,572,681
Director [Member] | Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash surrender value of life insurance	150,687	138,001	129,367
Present value of accumulated benefits Interest rate	4.50%	4.50%	5.25%
Accrual amounted Projected unit cost included in Employee and director benefit plans liabilities	$ 210,207	$ 206,650	$ 192,528