Allowance for Loan Losses - Analysis of Past Due and Nonaccrual Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	$ 4,307,366	$ 4,022,014	$ 3,015,165
Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	3,328,290	3,143,100	2,300,322
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	445,542	418,982	356,874
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	283,707	276,548	220,412
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	249,827	183,384	137,557
Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	509,508	515,262	385,426
Home equity loans | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	28,586	36,684	43,031
Other | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	82,656	55,721	25,155
Commercial | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	2,818,782	2,627,838	1,914,896
Commercial | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	46,861	62,088	69,627
Home equity lines of credit | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	334,300	326,577	288,688
Residential | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	202,966	121,296	$ 67,930
Originated
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	4,764	10,460
Financing Receivable, Recorded Investment, Current	3,482,931	2,970,996
Total	3,487,695	2,981,456
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,541	4,212
Financing Receivable, Recorded Investment, Current	2,591,777	2,250,565
Total	2,594,318	2,254,777
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,313	3,160
Financing Receivable, Recorded Investment, Current	391,380	347,973
Total	392,693	351,133
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	910	2,879
Financing Receivable, Recorded Investment, Current	261,671	225,756
Total	262,581	228,635
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	209
Financing Receivable, Recorded Investment, Current	238,103	146,702
Total	238,103	146,911
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,082	2,285
Financing Receivable, Recorded Investment, Current	371,019	341,511
Total	373,101	343,796
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Home equity loans | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	384	1,281
Financing Receivable, Recorded Investment, Current	24,959	31,379
Total	25,343	32,660
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Other | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	124	315
Financing Receivable, Recorded Investment, Current	67,772	33,705
Total	67,896	34,020
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0
Originated | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	459	1,927
Financing Receivable, Recorded Investment, Current	2,220,758	1,909,054
Total	2,221,217	1,910,981
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Commercial | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	46,422	56,208
Total	46,422	56,208
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Home equity lines of credit | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	805	1,564
Financing Receivable, Recorded Investment, Current	298,649	282,889
Total	299,454	284,453
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Originated | Residential | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	209
Financing Receivable, Recorded Investment, Current	191,681	90,494
Total	191,681	90,703
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
PNCI
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	3,421	6,908
Financing Receivable, Recorded Investment, Current	805,834	1,019,427
Total	809,255	1,026,335
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	19	0
PNCI | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,710	5,162
Financing Receivable, Recorded Investment, Current	724,528	873,031
Total	727,238	878,193
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0
PNCI | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	660	810
Financing Receivable, Recorded Investment, Current	50,984	65,391
Total	51,644	66,201
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	19	0
PNCI | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	51	936
Financing Receivable, Recorded Investment, Current	18,598	44,532
Total	18,649	45,468
PNCI | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	11,724	36,473
Total	11,724	36,473
PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	305	1,298
Financing Receivable, Recorded Investment, Current	134,689	168,494
Total	134,994	169,792
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0
PNCI | Home equity loans | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	51	74
Financing Receivable, Recorded Investment, Current	2,778	3,511
Total	2,829	3,585
PNCI | Other | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	19	160
Financing Receivable, Recorded Investment, Current	14,739	21,499
Total	14,758	21,659
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	19
PNCI | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,405	3,864
Financing Receivable, Recorded Investment, Current	589,839	704,537
Total	592,244	708,401
PNCI | Commercial | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	439	5,880
Total	439	5,880
PNCI | Home equity lines of credit | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	590	576
Financing Receivable, Recorded Investment, Current	33,467	40,381
Total	34,057	40,957
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0
PNCI | Residential | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	11,285	30,593
Total	11,285	30,593
Financing Receivables, 30 to 59 Days Past Due | Originated
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	850	5,617
Financing Receivables, 30 to 59 Days Past Due | Originated | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	90	2,106
Financing Receivables, 30 to 59 Days Past Due | Originated | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	156	2,249
Financing Receivables, 30 to 59 Days Past Due | Originated | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	604	1,053
Financing Receivables, 30 to 59 Days Past Due | Originated | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	209
Financing Receivables, 30 to 59 Days Past Due | Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	60	1,675
Financing Receivables, 30 to 59 Days Past Due | Originated | Home equity loans | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	36	1,043
Financing Receivables, 30 to 59 Days Past Due | Originated | Other | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	120	298
Financing Receivables, 30 to 59 Days Past Due | Originated | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	30	431
Financing Receivables, 30 to 59 Days Past Due | Originated | Commercial | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0
Financing Receivables, 30 to 59 Days Past Due | Originated | Home equity lines of credit | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	908
Financing Receivables, 30 to 59 Days Past Due | Originated | Residential | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	209
Financing Receivables, 30 to 59 Days Past Due | PNCI
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	406	3,871
Financing Receivables, 30 to 59 Days Past Due | PNCI | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	268	2,655
Financing Receivables, 30 to 59 Days Past Due | PNCI | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	138	538
Financing Receivables, 30 to 59 Days Past Due | PNCI | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	678
Financing Receivables, 30 to 59 Days Past Due | PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	1,009
Financing Receivables, 30 to 59 Days Past Due | PNCI | Home equity loans | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	51	74
Financing Receivables, 30 to 59 Days Past Due | PNCI | Other | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	160
Financing Receivables, 30 to 59 Days Past Due | PNCI | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	268	1,646
Financing Receivables, 30 to 59 Days Past Due | PNCI | Home equity lines of credit | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	87	304
Financing Receivables, 60 to 89 Days Past Due | Originated
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	807	1,999
Financing Receivables, 60 to 89 Days Past Due | Originated | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	201	1,332
Financing Receivables, 60 to 89 Days Past Due | Originated | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	309	88
Financing Receivables, 60 to 89 Days Past Due | Originated | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	297	579
Financing Receivables, 60 to 89 Days Past Due | Originated | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0
Financing Receivables, 60 to 89 Days Past Due | Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	65	132
Financing Receivables, 60 to 89 Days Past Due | Originated | Home equity loans | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	216	24
Financing Receivables, 60 to 89 Days Past Due | Originated | Other | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	17
Financing Receivables, 60 to 89 Days Past Due | Originated | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	136	1,200
Financing Receivables, 60 to 89 Days Past Due | Originated | Commercial | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0
Financing Receivables, 60 to 89 Days Past Due | Originated | Home equity lines of credit | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	93	47
Financing Receivables, 60 to 89 Days Past Due | Originated | Residential | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0
Financing Receivables, 60 to 89 Days Past Due | PNCI
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	565	1,449
Financing Receivables, 60 to 89 Days Past Due | PNCI | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	305	1,269
Financing Receivables, 60 to 89 Days Past Due | PNCI | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	260	35
Financing Receivables, 60 to 89 Days Past Due | PNCI | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	145
Financing Receivables, 60 to 89 Days Past Due | PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	305	133
Financing Receivables, 60 to 89 Days Past Due | PNCI | Other | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due		0
Financing Receivables, 60 to 89 Days Past Due | PNCI | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	1,136
Financing Receivables, 60 to 89 Days Past Due | PNCI | Home equity lines of credit | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	260	35
Financing Receivables, Equal to Greater than 90 Days Past Due | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due		296
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	3,107	2,844
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,250	774
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	848	823
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	9	1,247
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,957	478
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Home equity loans | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	132	214
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	4
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Other | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due		0
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	293
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Commercial | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Home equity lines of credit | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	712	609
Financing Receivables, Equal to Greater than 90 Days Past Due | Originated | Residential | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0
Financing Receivables, Equal to Greater than 90 Days Past Due | PNCI
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,450	1,588
Financing Receivables, Equal to Greater than 90 Days Past Due | PNCI | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,137	1,238
Financing Receivables, Equal to Greater than 90 Days Past Due | PNCI | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	262	237
Financing Receivables, Equal to Greater than 90 Days Past Due | PNCI | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	51	113
Financing Receivables, Equal to Greater than 90 Days Past Due | PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	156
Financing Receivables, Equal to Greater than 90 Days Past Due | PNCI | Other | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	19
Financing Receivables, Equal to Greater than 90 Days Past Due | PNCI | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,137	1,082
Financing Receivables, Equal to Greater than 90 Days Past Due | PNCI | Home equity lines of credit | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 243	$ 237