Loans - Summary of Loan Balances (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Total principal balance of loans owed, net of charge-offs	$ 4,152,540		$ 4,075,244
Unamortized net deferred loan fees	(9,050)		(9,868)
Discounts to principal balance of loans owed, net of charge-offs	(39,803)		(43,362)
Total loans, net of unamortized deferred loan fees and discounts	4,103,687		4,022,014	$ 3,146,313
Allowance for loan losses	(32,868)	$ (32,064)	(32,582)	(29,524)	$ (29,973)	$ (30,323)
Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,178,730		3,143,100	2,401,040
Allowance for loan losses	(14,675)	(14,830)	(15,620)	(13,598)	(13,665)	(13,758)
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	434,388		418,982	350,925
Allowance for loan losses	(8,552)	(8,341)	(8,375)	(7,137)	(7,718)	(8,227)
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	276,045		276,548	237,619
Allowance for loan losses	(6,745)	(6,078)	(6,090)	(6,378)	(6,392)	(6,512)
Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	214,524		183,384	156,729
Allowance for loan losses	(2,896)	(2,815)	(2,497)	(2,411)	(2,198)	(1,826)
Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	506,049		515,262	384,692
Allowance for loan losses	(2,576)	(2,500)	(2,676)	(1,991)	(2,170)	(2,317)
Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	32,345		36,684	40,986
Allowance for loan losses	(1,242)	(1,286)	(1,540)	(1,532)	(1,736)	(1,841)
Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	71,143		55,721	23,503
Allowance for loan losses	(1,451)	(1,040)	(793)	(557)	(570)	(586)
Commercial [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,672,681		2,627,838	2,016,348
Allowance for loan losses	(12,099)	(12,330)	(12,944)	(11,607)	(11,495)	(11,441)
Commercial [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	57,009		62,088	83,151
Allowance for loan losses	(358)	(407)	(663)	(977)	(847)	(642)
Home Equity Lines of Credit [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	330,900		326,577	286,436
Allowance for loan losses	(5,859)	(6,015)	(6,042)	(5,048)	(5,412)	(5,800)
Residential [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	157,515		121,296	73,578
Allowance for loan losses	(2,538)	$ (2,408)	(1,834)	$ (1,434)	$ (1,351)	$ (1,184)
Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total principal balance of loans owed, net of charge-offs	3,193,938		2,991,324
Unamortized net deferred loan fees	(9,050)		(9,868)
Total loans, net of unamortized deferred loan fees and discounts	3,184,888		2,981,456
Allowance for loan losses	(32,273)		(31,793)
Originated [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,354,722		2,254,777
Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	376,922		351,133
Originated [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	248,523		228,635
Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	204,721		146,911
Originated [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	348,737		343,796
Originated [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	29,041		32,660
Originated [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	53,340		34,020
Originated [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,005,985		1,910,981
Originated [Member] | Commercial [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	56,289		56,208
Originated [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	294,541		284,453
Originated [Member] | Residential [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	148,432		90,703
PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total principal balance of loans owed, net of charge-offs	940,627		1,062,655
Discounts to principal balance of loans owed, net of charge-offs	(33,323)		(36,320)
Total loans, net of unamortized deferred loan fees and discounts	907,304		1,026,335
Allowance for loan losses	(585)		(667)
PNCI [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	816,609		878,193
PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	55,908		66,201
PNCI [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	24,984		45,468
PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,803		36,473
PNCI [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	155,872		169,792
PNCI [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,875		3,585
PNCI [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	17,802		21,659
PNCI [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	660,737		708,401
PNCI [Member] | Commercial [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	720		5,880
PNCI [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	35,231		40,957
PNCI [Member] | Residential [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,083		30,593
PCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total principal balance of loans owed, net of charge-offs	17,975		21,265
Discounts to principal balance of loans owed, net of charge-offs	(6,480)		(7,042)
Total loans, net of unamortized deferred loan fees and discounts	11,495		14,223
Allowance for loan losses	(10)		(122)
PCI [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	7,399		10,130
PCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,558		1,648
PCI [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,538		2,445
PCI [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,440		1,674
PCI [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	429		439
PCI [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1		42
PCI [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	5,959		8,456
PCI [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 1,128		$ 1,167