Loans - Summary of Loan Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Total principal balance of loans owed, net of charge-offs	$ 4,082,558		$ 3,038,472
Unamortized net deferred loan fees	(10,190)		(8,651)
Discounts to principal balance of loans owed, net of charge-offs	(44,932)		(14,656)
Total loans, net of unamortized deferred loan fees and discounts	4,027,436		3,015,165	$ 2,931,613
Allowance for loan losses	(31,603)	$ (29,524)	(30,323)	(28,747)	$ (28,143)	$ (32,503)
Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,132,202		2,300,322	2,194,874
Allowance for loan losses	(15,279)	(13,598)	(13,758)	(13,588)	(12,614)	(14,265)
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	421,285		356,874	361,320
Allowance for loan losses	(7,514)	(7,137)	(8,227)	(8,302)	(9,155)	(10,310)
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	289,647		220,412	227,479
Allowance for loan losses	(6,224)	(6,378)	(6,512)	(4,802)	(4,729)	(5,831)
Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	184,302		137,557	147,940
Allowance for loan losses	(2,586)	(2,411)	(1,826)	(2,055)	(1,645)	(2,097)
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	524,414		385,426	391,072
Allowance for loan losses	(2,400)	(1,991)	(2,317)	(2,218)	(2,495)	(2,748)
Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	40,693		43,031	45,033
Allowance for loan losses	(1,593)	(1,532)	(1,841)	(1,991)	(2,354)	(2,644)
Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	49,456		25,155	26,781
Allowance for loan losses	(700)	(557)	(586)	(590)	(645)	(622)
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	2,607,788		1,914,896	1,803,802
Allowance for loan losses	(12,879)	(11,607)	(11,441)	(11,370)	(10,119)	(11,517)
Commercial [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	69,728		69,627	72,820
Allowance for loan losses	(960)	(977)	(642)	(625)	(466)	(680)
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	331,136		288,688	289,506
Allowance for loan losses	(5,221)	(5,048)	(5,800)	(5,721)	(6,156)	(7,044)
Residential [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	114,574		67,930	75,120
Allowance for loan losses	(1,626)	$ (1,434)	(1,184)	$ (1,430)	$ (1,179)	$ (1,417)
Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total principal balance of loans owed, net of charge-offs	2,940,897		2,699,053
Unamortized net deferred loan fees	(10,190)		(8,651)
Total loans, net of unamortized deferred loan fees and discounts	2,930,707		2,690,402
Allowance for loan losses	(30,927)		(29,122)
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	2,204,119		2,011,032
Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	346,806		332,649
Originated [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	234,741		209,437
Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	145,041		137,284
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	340,515		320,522
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	35,556		39,848
Originated [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	26,294		22,859
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	1,863,604		1,690,510
Originated [Member] | Commercial [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	63,508		69,364
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	284,956		269,942
Originated [Member] | Residential [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	81,533		67,920
PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total principal balance of loans owed, net of charge-offs	1,120,654		316,238
Discounts to principal balance of loans owed, net of charge-offs	(37,723)		(7,035)
Total loans, net of unamortized deferred loan fees and discounts	1,082,931		309,203
Allowance for loan losses	(566)		(929)
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	918,500		279,342
PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	72,691		21,197
PNCI [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	52,479		8,391
PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	39,261		273
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	182,201		63,519
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	4,690		2,698
PNCI [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	23,120		2,251
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	736,299		215,823
PNCI [Member] | Commercial [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	6,220		263
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	44,881		16,248
PNCI [Member] | Residential [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	33,041		10
PCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total principal balance of loans owed, net of charge-offs	21,007		23,181
Discounts to principal balance of loans owed, net of charge-offs	(7,209)		(7,621)
Total loans, net of unamortized deferred loan fees and discounts	13,798		15,560
Allowance for loan losses	(110)		(272)
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	9,583		9,948
PCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,788		3,028
PCI [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,427		2,584
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	1,698		1,385
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	447		485
PCI [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	42		45
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	7,885		8,563
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	$ 1,299		$ 2,498