Allowance for Loan Losses - Aging Analysis of Recorded Investment for Loans by Portfolio and Segment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	$ 7,612	$ 14,246
Non- Accrual	3,121	3,293
Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	12,406	10,647
Non- Accrual	8,515	7,832
30 to 59 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	4,491	7,326
30 to 59 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	3,848	2,771
60 to 89 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	3,627
60 to 89 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	44
90 Days or More Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
90 Days or More Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	43	0
Total real estate loans | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	7,612	7,099
Non- Accrual	3,121	3,293
Total real estate loans | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	9,776	10,152
Non- Accrual	8,121	7,438
Total real estate loans | 30 to 59 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	4,491	2,346
Total real estate loans | 30 to 59 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	1,612	2,670
Total real estate loans | 60 to 89 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	1,460
Total real estate loans | 60 to 89 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	44
Total real estate loans | 90 Days or More Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Total real estate loans | 90 Days or More Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	43	0
Commercial real estate | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	324
Non- Accrual	0	0
Commercial real estate | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	3,847	4,253
Non- Accrual	3,847	3,893
Commercial real estate | 30 to 59 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	324
Commercial real estate | 30 to 59 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	360
Commercial real estate | 60 to 89 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Commercial real estate | 60 to 89 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Commercial real estate | 90 Days or More Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Commercial real estate | 90 Days or More Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Owner-occupied commercial real estate | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	983	993
Non- Accrual	0	0
Owner-occupied commercial real estate | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	492	784
Non- Accrual	492	494
Owner-occupied commercial real estate | 30 to 59 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	983	843
Owner-occupied commercial real estate | 30 to 59 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	290
Owner-occupied commercial real estate | 60 to 89 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	150
Owner-occupied commercial real estate | 60 to 89 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Owner-occupied commercial real estate | 90 Days or More Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Owner-occupied commercial real estate | 90 Days or More Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
1-4 single family residential | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	6,248	5,656
Non- Accrual	2,995	3,167
1-4 single family residential | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	2,189	2,267
Non- Accrual	1,298	1,331
1-4 single family residential | 30 to 59 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	3,253	1,179
1-4 single family residential | 30 to 59 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	848	892
1-4 single family residential | 60 to 89 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	1,310
1-4 single family residential | 60 to 89 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	44
1-4 single family residential | 90 Days or More Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
1-4 single family residential | 90 Days or More Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	43	0
Construction, land and development | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Non- Accrual	0	0
Construction, land and development | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Non- Accrual	0	0
Construction, land and development | 30 to 59 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Construction, land and development | 30 to 59 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Construction, land and development | 60 to 89 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Construction, land and development | 60 to 89 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Construction, land and development | 90 Days or More Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Construction, land and development | 90 Days or More Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Home equity loans and lines of credit | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	381	126
Non- Accrual	126	126
Home equity loans and lines of credit | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	3,248	2,848
Non- Accrual	2,484	1,720
Home equity loans and lines of credit | 30 to 59 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	255	0
Home equity loans and lines of credit | 30 to 59 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	764	1,128
Home equity loans and lines of credit | 60 to 89 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Home equity loans and lines of credit | 60 to 89 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Home equity loans and lines of credit | 90 Days or More Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Home equity loans and lines of credit | 90 Days or More Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Total other loans | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	7,147
Non- Accrual	0	0
Total other loans | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	2,630	495
Non- Accrual	394	394
Total other loans | 30 to 59 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	4,980
Total other loans | 30 to 59 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	2,236	101
Total other loans | 60 to 89 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	2,167
Total other loans | 60 to 89 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Total other loans | 90 Days or More Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Total other loans | 90 Days or More Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Consumer | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Non- Accrual	0	0
Consumer | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Non- Accrual	0	0
Consumer | 30 to 59 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Consumer | 30 to 59 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Consumer | 60 to 89 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Consumer | 60 to 89 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Consumer | 90 Days or More Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Consumer | 90 Days or More Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Commercial and industrial | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	7,147
Non- Accrual	0	0
Commercial and industrial | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	2,630	495
Non- Accrual	394	394
Commercial and industrial | 30 to 59 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	4,980
Commercial and industrial | 30 to 59 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	2,236	101
Commercial and industrial | 60 to 89 Days Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	2,167
Commercial and industrial | 60 to 89 Days Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Commercial and industrial | 90 Days or More Past Due | New Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	0	0
Commercial and industrial | 90 Days or More Past Due | Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables past due	$ 0	$ 0