Allowance for Loan Losses - Analysis of Past Due and Nonaccrual Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 8,455	$ 8,687
Current	2,598,084	2,347,640
Total orig. loans	2,606,539	2,356,327
> 90 Days and still accruing	0	0
Nonaccrual loans	11,688	12,894
PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,116	2,600
Current	305,954	375,622
Total orig. loans	309,070	378,222
> 90 Days and still accruing	0	0
Nonaccrual loans	3,026	2,834
30-59 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	4,831	4,354
30-59 Days [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	178	1,896
60-89 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,579	2,586
60-89 Days [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,911
> 90 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,045	1,747
> 90 Days [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,027	704
Mortgage Loans on Real Estate [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,467	821
Current	320,385	283,718
Total orig. loans	321,852	284,539
> 90 Days and still accruing	0	0
Nonaccrual loans	1,037	255
Mortgage Loans on Real Estate [Member] | Residential [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,219	1,531
Current	66,596	80,804
Total orig. loans	67,815	82,335
> 90 Days and still accruing	0	0
Nonaccrual loans	1,089	194
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,778	1,920
Current	1,585,012	1,423,908
Total orig. loans	1,588,790	1,425,828
> 90 Days and still accruing	0	0
Nonaccrual loans	6,954	7,651
Mortgage Loans on Real Estate [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,599	154
Current	205,242	246,337
Total orig. loans	206,841	246,491
> 90 Days and still accruing	0	0
Nonaccrual loans	1,599	1,826
Mortgage Loans on Real Estate [Member] | 30-59 Days [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	721	552
Mortgage Loans on Real Estate [Member] | 30-59 Days [Member] | Residential [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	39	1,510
Mortgage Loans on Real Estate [Member] | 30-59 Days [Member] | Commercial [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,241	317
Mortgage Loans on Real Estate [Member] | 30-59 Days [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		73
Mortgage Loans on Real Estate [Member] | 60-89 Days [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	403	269
Mortgage Loans on Real Estate [Member] | 60-89 Days [Member] | Residential [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	159
Mortgage Loans on Real Estate [Member] | 60-89 Days [Member] | Commercial [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	793	1,387
Mortgage Loans on Real Estate [Member] | 60-89 Days [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,599
Mortgage Loans on Real Estate [Member] | > 90 Days [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	343
Mortgage Loans on Real Estate [Member] | > 90 Days [Member] | Residential [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,021	21
Mortgage Loans on Real Estate [Member] | > 90 Days [Member] | Commercial [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	744	216
Mortgage Loans on Real Estate [Member] | > 90 Days [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		81
Consumer [Member] | Home Equity Lines of Credit [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,076	1,441
Current	268,394	262,149
Total orig. loans	269,470	263,590
> 90 Days and still accruing	0	0
Nonaccrual loans	534	1,211
Consumer [Member] | Home Equity Lines of Credit [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	229	863
Current	16,855	20,902
Total orig. loans	17,084	21,765
> 90 Days and still accruing	0	0
Nonaccrual loans	276	742
Consumer [Member] | 30-59 Days [Member] | Home Equity Lines of Credit [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	695	754
Consumer [Member] | 30-59 Days [Member] | Home Equity Lines of Credit [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	100	274
Consumer [Member] | 60-89 Days [Member] | Home Equity Lines of Credit [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	299
Consumer [Member] | 60-89 Days [Member] | Home Equity Lines of Credit [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	129
Consumer [Member] | > 90 Days [Member] | Home Equity Lines of Credit [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	82	687
Consumer [Member] | > 90 Days [Member] | Home Equity Lines of Credit [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		589
Consumer [Member] | Home Equity Loans [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,130	1,225
Current	40,356	39,511
Total orig. loans	41,486	40,736
> 90 Days and still accruing	0	0
Nonaccrual loans	1,199	803
Consumer [Member] | Home Equity Loans [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	45	52
Current	2,765	3,712
Total orig. loans	2,810	3,764
> 90 Days and still accruing	0	0
Nonaccrual loans	50	67
Consumer [Member] | Home Equity Loans [Member] | 30-59 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	662	646
Consumer [Member] | Home Equity Loans [Member] | 30-59 Days [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	27	39
Consumer [Member] | Home Equity Loans [Member] | 60-89 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		395
Consumer [Member] | Home Equity Loans [Member] | 60-89 Days [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	18
Consumer [Member] | Home Equity Loans [Member] | > 90 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	468	184
Consumer [Member] | Home Equity Loans [Member] | > 90 Days [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		13
Consumer [Member] | Other Consumer [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	234	61
Current	24,201	28,106
Total orig. loans	24,435	28,167
> 90 Days and still accruing	0	0
Nonaccrual loans	13	33
Consumer [Member] | Other Consumer [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	17
Current	2,285	2,534
Total orig. loans	2,302	2,534
> 90 Days and still accruing	0	0
Nonaccrual loans	6	5
Consumer [Member] | Other Consumer [Member] | 30-59 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	150	16
Consumer [Member] | Other Consumer [Member] | 30-59 Days [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	12
Consumer [Member] | Other Consumer [Member] | 60-89 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	83	30
Consumer [Member] | Other Consumer [Member] | 60-89 Days [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	5
Consumer [Member] | Other Consumer [Member] | > 90 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1	15
Consumer and Industrial [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	770	1,866
Current	215,176	198,869
Total orig. loans	215,946	200,735
> 90 Days and still accruing	0	0
Nonaccrual loans	1,951	2,930
Consumer and Industrial [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	7
Current	8,831	12,321
Total orig. loans	8,838	12,321
> 90 Days and still accruing	0	0
Nonaccrual loans	6
Consumer and Industrial [Member] | 30-59 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	362	1,148
Consumer and Industrial [Member] | 60-89 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1	84
Consumer and Industrial [Member] | 60-89 Days [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1
Consumer and Industrial [Member] | > 90 Days [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	407	634
Consumer and Industrial [Member] | > 90 Days [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	6
Construction [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		932
Current	75,108	53,681
Total orig. loans	75,108	54,613
> 90 Days and still accruing	0	0
Nonaccrual loans		11
Construction [Member] | Residential [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	12	141
Total orig. loans	12	141
> 90 Days and still accruing	0	0
Construction [Member] | Commercial [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		421
Current	69,452	57,698
Total orig. loans	69,452	58,119
> 90 Days and still accruing	0	0
Construction [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,368	8,871
Total orig. loans	3,368	8,871
> 90 Days and still accruing	$ 0	0
Construction [Member] | 30-59 Days [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		921
Construction [Member] | 60-89 Days [Member] | Commercial [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		421
Construction [Member] | > 90 Days [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		$ 11