Allowance for Loan Losses - Analysis of Past Due and Nonaccrual Loans (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total	$ 4,182,348	$ 4,027,436	$ 4,022,014
Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Total	3,247,156	3,132,202	3,143,100
Consumer
Financing Receivable, Past Due [Line Items]
Total	442,539	421,285	418,982
Commercial
Financing Receivable, Past Due [Line Items]
Total	278,458	289,647	276,548
Construction
Financing Receivable, Past Due [Line Items]
Total	214,195	184,302	183,384
Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Total	505,794	524,414	515,262
Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Total	30,370	40,693	36,684
Other | Consumer
Financing Receivable, Past Due [Line Items]
Total	80,297	49,456	55,721
Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Total	2,741,362	2,607,788	2,627,838
Commercial loans | Construction
Financing Receivable, Past Due [Line Items]
Total	45,161	69,728	62,088
Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Total	331,872	331,136	326,577
Residential | Construction
Financing Receivable, Past Due [Line Items]
Total	169,034	114,574	121,296
Originated
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	18,737	25,607	27,385
Past due	5,728		10,460
Current	3,304,815		2,970,996
Total	3,310,543		2,981,456
> 90 Days and Still Accruing	36
Average recorded investment	21,760	23,292	24,713
Originated | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	10,467	16,738	17,675
Past due	1,707		4,212
Current	2,463,248		2,250,565
Total	2,464,955		2,254,777
Average recorded investment	13,310	15,432	16,632
Originated | Consumer
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	3,510	4,001	4,277
Past due	2,567		3,160
Current	383,031		347,973
Total	385,598		351,133
> 90 Days and Still Accruing	6
Average recorded investment	3,681	3,166	3,422
Originated | Commercial
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	4,760	4,868	5,433
Past due	1,454		2,879
Current	254,925		225,756
Total	256,379		228,635
> 90 Days and Still Accruing	30
Average recorded investment	4,769	4,626	4,654
Originated | Construction
Financing Receivable, Past Due [Line Items]
Past due	0		209
Current	203,611		146,702
Total	203,611		146,911
Average recorded investment		68	5
Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	4,461	4,185	4,594
Past due	1,542		2,285
Current	354,104		341,511
Total	355,646		343,796
Average recorded investment	4,214	3,883	3,517
Originated | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	2,157	2,554	2,374
Past due	495		1,281
Current	26,539		31,379
Total	27,034		32,660
> 90 Days and Still Accruing	6
Average recorded investment	1,973	1,753	1,520
Originated | Other | Consumer
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	74	3	3
Past due	276		315
Current	63,877		33,705
Total	64,153		34,020
> 90 Days and Still Accruing	0
Average recorded investment	46	3	17
Originated | Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	6,006	12,553	13,081
Past due	165		1,927
Current	2,109,144		1,909,054
Total	2,109,309		1,910,981
Average recorded investment	9,096	11,549	13,115
Originated | Commercial loans | Construction
Financing Receivable, Past Due [Line Items]
Current	44,704		56,208
Total	44,704		56,208
Originated | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	1,279	1,444	1,900
Past due	1,796		1,564
Current	292,615		282,889
Total	294,411		284,453
Average recorded investment	1,662	1,410	1,885
Originated | Residential | Construction
Financing Receivable, Past Due [Line Items]
Past due	0		209
Current	158,907		90,494
Total	158,907		90,703
Average recorded investment		68	5
PNCI
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	5,626	3,945	4,990
Past due	2,029		6,908
Current	859,471		1,019,427
Total	861,500		1,026,335
Average recorded investment	4,383	3,355	3,691
PNCI | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	4,214	2,557	3,485
Past due	1,244		5,162
Current	774,411		873,031
Total	775,655		878,193
Average recorded investment	2,981	1,902	2,242
PNCI | Consumer
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	1,231	1,388	1,385
Past due	611		810
Current	55,081		65,391
Total	55,692		66,201
Average recorded investment	1,255	1,453	1,389
PNCI | Commercial
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	181		120
Past due	174		936
Current	19,395		44,532
Total	19,569		45,468
Average recorded investment	147		60
PNCI | Construction
Financing Receivable, Past Due [Line Items]
Past due	0
Current	10,584		36,473
Total	10,584		36,473
PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	959	1,302	375
Past due	295		1,298
Current	148,436		168,494
Total	148,731		169,792
Average recorded investment	621	1,275	529
PNCI | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	395	296	252
Past due	246		74
Current	2,669		3,511
Total	2,915		3,585
Average recorded investment	308	287	155
PNCI | Other | Consumer
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	95	240	106
Past due	61		160
Current	16,081		21,499
Total	16,142		21,659
Average recorded investment	103	257	114
PNCI | Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	3,255	1,255	3,110
Past due	949		3,864
Current	625,975		704,537
Total	626,924		708,401
Average recorded investment	2,360	627	1,713
PNCI | Commercial loans | Construction
Financing Receivable, Past Due [Line Items]
Past due	0
Current	457		5,880
Total	457		5,880
PNCI | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Unpaid principal balance	741	852	1,027
Past due	304		576
Current	36,331		40,381
Total	36,635		40,957
Average recorded investment	844	$ 909	1,120
PNCI | Residential | Construction
Financing Receivable, Past Due [Line Items]
Current	10,127		30,593
Total	10,127		30,593
30-59 days | Originated
Financing Receivable, Past Due [Line Items]
Past due	2,638		5,617
30-59 days | Originated | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	406		2,106
30-59 days | Originated | Consumer
Financing Receivable, Past Due [Line Items]
Past due	1,277		2,249
30-59 days | Originated | Commercial
Financing Receivable, Past Due [Line Items]
Past due	955		1,053
30-59 days | Originated | Construction
Financing Receivable, Past Due [Line Items]
Past due	0		209
30-59 days | Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	289		1,675
30-59 days | Originated | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Past due	253		1,043
30-59 days | Originated | Other | Consumer
Financing Receivable, Past Due [Line Items]
Past due	134		298
30-59 days | Originated | Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	117		431
30-59 days | Originated | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Past due	890		908
30-59 days | Originated | Residential | Construction
Financing Receivable, Past Due [Line Items]
Past due	0		209
30-59 days | PNCI
Financing Receivable, Past Due [Line Items]
Past due	236		3,871
30-59 days | PNCI | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	0		2,655
30-59 days | PNCI | Consumer
Financing Receivable, Past Due [Line Items]
Past due	236		538
30-59 days | PNCI | Commercial
Financing Receivable, Past Due [Line Items]
Past due	0		678
30-59 days | PNCI | Construction
Financing Receivable, Past Due [Line Items]
Past due	0
30-59 days | PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	0		1,009
30-59 days | PNCI | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Past due	0		74
30-59 days | PNCI | Other | Consumer
Financing Receivable, Past Due [Line Items]
Past due	54		160
30-59 days | PNCI | Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	0		1,646
30-59 days | PNCI | Commercial loans | Construction
Financing Receivable, Past Due [Line Items]
Past due	0
30-59 days | PNCI | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Past due	182		304
60-89 days | Originated
Financing Receivable, Past Due [Line Items]
Past due	866		1,999
60-89 days | Originated | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	114		1,332
60-89 days | Originated | Consumer
Financing Receivable, Past Due [Line Items]
Past due	525		88
60-89 days | Originated | Commercial
Financing Receivable, Past Due [Line Items]
Past due	227		579
60-89 days | Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	66		132
60-89 days | Originated | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Past due	105		24
60-89 days | Originated | Other | Consumer
Financing Receivable, Past Due [Line Items]
Past due	138		17
60-89 days | Originated | Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	48		1,200
60-89 days | Originated | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Past due	282		47
60-89 days | PNCI
Financing Receivable, Past Due [Line Items]
Past due	195		1,449
60-89 days | PNCI | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	52		1,269
60-89 days | PNCI | Consumer
Financing Receivable, Past Due [Line Items]
Past due	143		35
60-89 days | PNCI | Commercial
Financing Receivable, Past Due [Line Items]
Past due	0		145
60-89 days | PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	52		133
60-89 days | PNCI | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Past due	14
60-89 days | PNCI | Other | Consumer
Financing Receivable, Past Due [Line Items]
Past due	7
60-89 days | PNCI | Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	0		1,136
60-89 days | PNCI | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Past due	122		35
> 90 days | Originated
Financing Receivable, Past Due [Line Items]
Past due	2,224		2,844
> 90 days | Originated | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	1,187		774
> 90 days | Originated | Consumer
Financing Receivable, Past Due [Line Items]
Past due	765		823
> 90 days | Originated | Commercial
Financing Receivable, Past Due [Line Items]
Past due	272		1,247
> 90 days | Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	1,187		478
> 90 days | Originated | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Past due	137		214
> 90 days | Originated | Other | Consumer
Financing Receivable, Past Due [Line Items]
Past due	4
> 90 days | Originated | Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	0		296
> 90 days | Originated | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Past due	624		609
> 90 days | PNCI
Financing Receivable, Past Due [Line Items]
Past due	1,598		1,588
> 90 days | PNCI | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	1,192		1,238
> 90 days | PNCI | Consumer
Financing Receivable, Past Due [Line Items]
Past due	232		237
> 90 days | PNCI | Commercial
Financing Receivable, Past Due [Line Items]
Past due	174		113
> 90 days | PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	243		156
> 90 days | PNCI | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Past due	232
> 90 days | PNCI | Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	949		1,082
> 90 days | PNCI | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Past due	$ 0		$ 237