Fair Value Disclosures - Assets, Liabilities and Off-balance Sheet Items Not Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Held-to-maturity securities	$ 55,320	$ 24,871
Total assets	1,958	2,138
Deposits:
Time deposits	574,749	626,189
Subordinated notes	94,331	94,087
Fair Value [Member]
Assets:
Cash and short-term interest-earning assets	75,409	57,825
Held-to-maturity securities	55,320	24,871
Loans held for sale	1,676	5,943
Net loans and leases held for investment	3,547,451	3,193,886
Servicing rights	10,046	9,548
Total assets	3,689,902	3,292,073
Deposits:
Demand and savings deposits, non-maturity	2,980,170	2,631,378
Time deposits	574,737	628,096
Total deposits	3,554,907	3,259,474
Short-term borrowings	105,431	195,572
Long-term debt	156,834	130,157
Subordinated notes	98,075	95,188
Total liabilities	3,915,247	3,680,391
Off-Balance-Sheet:
Commitments to extend credit	(2,414)	(2,218)
Fair Value [Member] | Level 1 [Member]
Assets:
Cash and short-term interest-earning assets	75,409	57,825
Held-to-maturity securities	0	0
Loans held for sale	0	0
Net loans and leases held for investment	0	0
Servicing rights	0	0
Total assets	75,409	57,825
Deposits:
Demand and savings deposits, non-maturity	2,980,170	2,631,378
Time deposits	0	0
Total deposits	2,980,170	2,631,378
Short-term borrowings	0	0
Long-term debt	0	0
Subordinated notes	0	0
Total liabilities	2,980,170	2,631,378
Off-Balance-Sheet:
Commitments to extend credit	0	0
Fair Value [Member] | Level 2 [Member]
Assets:
Cash and short-term interest-earning assets	0	0
Held-to-maturity securities	55,320	24,871
Loans held for sale	1,676	5,943
Net loans and leases held for investment	0	0
Servicing rights	0	0
Total assets	56,996	30,814
Deposits:
Demand and savings deposits, non-maturity	0	0
Time deposits	574,737	628,096
Total deposits	574,737	628,096
Short-term borrowings	105,431	195,572
Long-term debt	156,834	130,157
Subordinated notes	98,075	95,188
Total liabilities	935,077	1,049,013
Off-Balance-Sheet:
Commitments to extend credit	(2,414)	(2,218)
Fair Value [Member] | Level 3 [Member]
Assets:
Cash and short-term interest-earning assets	0	0
Held-to-maturity securities	0	0
Loans held for sale	0	0
Net loans and leases held for investment	3,547,451	3,193,886
Servicing rights	10,046	9,548
Total assets	3,557,497	3,203,434
Deposits:
Demand and savings deposits, non-maturity	0	0
Time deposits	0	0
Total deposits	0	0
Short-term borrowings	0	0
Long-term debt	0	0
Subordinated notes	0	0
Total liabilities	0	0
Off-Balance-Sheet:
Commitments to extend credit	0	0
Carrying Amount [Member]
Assets:
Cash and short-term interest-earning assets	75,409	57,825
Held-to-maturity securities	55,564	24,881
Federal Home Loan Bank, Federal Reserve Bank and other stock	27,204	24,869
Loans held for sale	1,642	5,890
Net loans and leases held for investment	3,566,953	3,222,569
Servicing rights	6,573	6,485
Total assets	3,733,345	3,342,519
Deposits:
Demand and savings deposits, non-maturity	2,980,170	2,631,378
Time deposits	574,749	626,189
Total deposits	3,554,919	3,257,567
Short-term borrowings	105,431	196,171
Long-term debt	155,828	127,522
Subordinated notes	94,331	94,087
Total liabilities	3,910,509	3,675,347
Off-Balance-Sheet:
Commitments to extend credit	$ 0	$ 0