Allowance for Loan Losses - Summary of Changes in Allowance for Loan Losses by Loan Class and Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 47,145
Total provision	(2,076)	$ (1,643)
Total charge-offs	(43)	(252)
Total recoveries	35	143
Ending balance	49,213	39,431	$ 47,145
Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	(18)	88
Total charge-offs	(36)	(7)
Total recoveries	0	29
Ending balance	1,113	1,344
Acquired Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	2	319
Total charge-offs	(7)	(14)
Total recoveries	30	114
Ending balance	1,910	2,725
New Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	(2,060)	(2,050)
Total charge-offs	0	(231)
Total recoveries	5	0
Ending balance	44,690	35,362
Unallocated Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	0		(1,500)
Ending balance	1,500
Unallocated Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending balance	0
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	(322)	42
Total charge-offs	0	(131)
Total recoveries	5	14
Ending balance	14,197	9,964
Commercial real estate | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	1	32
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	337	344
Commercial real estate | Acquired Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	1,448
Total provision	(31)	1,352
Total charge-offs	0	0
Total recoveries	0	14
Ending balance	1,479	917	1,448
Commercial real estate | New Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	(292)	(1,342)
Total charge-offs	0	(131)
Total recoveries	5	0
Ending balance	12,381	8,703
Commercial real estate | New Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0
Ending balance	0		0
Commercial real estate | Unallocated Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	0
Ending balance	0
Owner-occupied commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	(413)	(114)
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	3,778	2,711
Owner-occupied commercial real estate | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	(3)	0
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	58	61
Owner-occupied commercial real estate | Acquired Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0
Total provision	0	0
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	0	0	0
Owner-occupied commercial real estate | New Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	(410)	(114)
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	3,720	2,650
Owner-occupied commercial real estate | New Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0
Ending balance	0		0
Owner-occupied commercial real estate | Unallocated Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	0
Ending balance	0
1-4 single family residential
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	485	(534)
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	7,493	7,913
1-4 single family residential | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	12	58
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	172	243
1-4 single family residential | Acquired Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	25
Total provision	(15)	(2)
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	40	31	25
1-4 single family residential | New Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	488	(590)
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	7,281	7,639
1-4 single family residential | New Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0
Ending balance	0		0
1-4 single family residential | Unallocated Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	0
Ending balance	0
Construction, land and development
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	70	(241)
Total charge-offs	0	0
Total recoveries	13	0
Ending balance	4,288	4,918
Construction, land and development | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	0	3
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	36	44
Construction, land and development | Acquired Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	145
Total provision	15	(66)
Total charge-offs	0	0
Total recoveries	13	0
Ending balance	143	305	145
Construction, land and development | New Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	55	(178)
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	4,109	4,569
Construction, land and development | New Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0
Ending balance	0		0
Construction, land and development | Unallocated Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	0
Ending balance	0
Home equity loans and lines of credit
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	58	(85)
Total charge-offs	0	(7)
Total recoveries	0	0
Ending balance	616	726
Home equity loans and lines of credit | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	13	(40)
Total charge-offs	0	(7)
Total recoveries	0	0
Ending balance	186	276
Home equity loans and lines of credit | Acquired Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0
Total provision	0	0
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	0	0	0
Home equity loans and lines of credit | New Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	45	(45)
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	430	450
Home equity loans and lines of credit | New Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0
Ending balance	0		0
Home equity loans and lines of credit | Unallocated Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	0
Ending balance	0
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	(1,992)	(822)
Total charge-offs	(36)	(114)
Total recoveries	17	0
Ending balance	17,114	12,953
Commercial and industrial | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	(44)	6
Total charge-offs	(36)	0
Total recoveries	0	0
Ending balance	321	370
Commercial and industrial | Acquired Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	126
Total provision	31	(1,047)
Total charge-offs	0	(14)
Total recoveries	17	0
Ending balance	112	1,310	126
Commercial and industrial | New Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	(1,979)	219
Total charge-offs	0	(100)
Total recoveries	0	0
Ending balance	16,681	11,273
Commercial and industrial | New Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0
Ending balance	0		0
Commercial and industrial | Unallocated Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	0
Ending balance	0
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	38	111
Total charge-offs	(7)	0
Total recoveries	0	129
Ending balance	227	246
Consumer | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	3	29
Total charge-offs	0	0
Total recoveries	0	29
Ending balance	3	6
Consumer | Acquired Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	145
Total provision	2	82
Total charge-offs	(7)	0
Total recoveries	0	100
Ending balance	136	162	145
Consumer | New Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	33	0
Total charge-offs	0	0
Total recoveries	0	0
Ending balance	88	$ 78
Consumer | New Loans | ASC 310-30 Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0
Ending balance	0		$ 0
Consumer | Unallocated Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total provision	0
Ending balance	$ 0