Loans and Allowance for Loan Losses - Schedule of Details on Loans of Company (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Mortgage loans on real estate
Total mortgage loans on real estate	$ 1,391,902	$ 1,273,553
Commercial loans	27,278	30,000
Agricultural loans	1,524	1,670
Consumer installment loans
Total consumer installment loans	41,770	41,025
Other loans	9,700	10,530
Gross Loan and Leases Receivable	1,472,174	1,356,778
Net deferred loan fees	(5,138)	(4,341)
Total loans	1,467,036	1,352,437
Less: Allowance for loan losses	(22,702)	(22,572)	$ (22,935)
Net loans	1,444,334	1,329,865
Residential 1-4 Family [Member]
Mortgage loans on real estate
Total mortgage loans on real estate	357,360	350,758
Consumer installment loans
Gross Loan and Leases Receivable	357,360	350,758
Less: Allowance for loan losses	(5,688)	(5,582)	(4,935)
Multifamily [Member]
Mortgage loans on real estate
Total mortgage loans on real estate	53,956	31,242
Consumer installment loans
Gross Loan and Leases Receivable	53,956	31,242
Less: Allowance for loan losses	(297)	(172)	(77)
Commercial Real Estate [Member]
Mortgage loans on real estate
Total mortgage loans on real estate	603,283	564,965
Consumer installment loans
Gross Loan and Leases Receivable	603,283	564,965
Less: Allowance for loan losses	(8,591)	(9,578)	(10,918)
Construction [Member]
Mortgage loans on real estate
Total mortgage loans on real estate	286,381	245,830
Consumer installment loans
Gross Loan and Leases Receivable	286,381	245,830
Less: Allowance for loan losses	(6,509)	(5,578)	(5,159)
Farmland [Member]
Mortgage loans on real estate
Total mortgage loans on real estate	34,911	30,236
Consumer installment loans
Gross Loan and Leases Receivable	34,911	30,236
Less: Allowance for loan losses	(611)	(795)	(618)
Second Mortgages [Member]
Mortgage loans on real estate
Total mortgage loans on real estate	8,053	9,026
Consumer installment loans
Gross Loan and Leases Receivable	8,053	9,026
Less: Allowance for loan losses	(88)	(61)	(205)
Equity Lines of Credit [Member]
Mortgage loans on real estate
Total mortgage loans on real estate	47,958	41,496
Consumer installment loans
Gross Loan and Leases Receivable	47,958	41,496
Less: Allowance for loan losses	(348)	(304)	$ (300)
Personal [Member]
Consumer installment loans
Total consumer installment loans	38,768	37,745
Credit Cards [Member]
Consumer installment loans
Total consumer installment loans	$ 3,002	$ 3,280

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount of deferred interest and fee income, unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of acquired loans. Excludes amounts for loans and leases covered under loss sharing agreements.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Reflects the aggregate gross carrying amount of all commercial loans and leases held in portfolio, prior to deducting deferred income and the allowance for losses on commercial loans and leases.

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Reflects the aggregate gross carrying amount of all consumer loans and leases held in portfolio, prior to deducting deferred income and the allowance for losses on consumer loans and leases.

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Reflects the aggregate gross carrying amount of all loans and leases held in portfolio, not otherwise categorized (such as commercial, consumer, or foreign), prior to deducting deferred income and the allowance for losses on loans and leases, not otherwise categorized (such as commercial, consumer, or foreign).

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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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Total carrying amount of mortgage loans as of the balance sheet date.

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Agriculture loans.

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Consumer installment loans.

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