Loans - Summary of Loan Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Mortgage loans on real estate:
Total mortgage loan on real estate	$ 1,757,082		$ 1,615,359
Consumer:
Total consumer loans	403,081		417,084
Commercial	199,330		174,945
Construction:
Total construction	110,073		75,136
Total loans, net of deferred loan fees and discounts	2,469,566		2,282,524	$ 1,765,871
Total principal balance of loans owed, net of charge-offs	2,504,104		2,322,853
Unamortized net deferred loan fees	(6,232)		(4,793)
Discounts to principal balance of loans owed, net of charge-offs	(28,306)		(35,536)
Total loans, net of unamortized deferred loan fees and discounts	2,469,566		2,282,524	1,765,871
Noncovered loans	2,463,680		2,260,256
Covered loans	5,886		22,268
Total loans, net of unamortized deferred loan fees and discounts	2,469,566		2,282,524	1,765,871
Allowance for loan losses	36,518	$ 35,455	36,585	37,920	$ 39,968	$ 38,245
Residential 1-4 family [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	302,105		279,420
Home equity loans [Member]
Consumer:
Total consumer loans	38,365		31,314
Auto Indirect [Member]
Consumer:
Total consumer loans			112
Other [Member]
Consumer:
Total consumer loans	33,027		33,074
Commercial [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	1,454,977		1,335,939
Construction:
Total construction	64,006		36,518
Home equity lines of credit [Member]
Consumer:
Total consumer loans	331,689		352,584
Construction:
Total loans, net of deferred loan fees and discounts	331,689		352,584	321,942
Total loans, net of unamortized deferred loan fees and discounts	331,689		352,584	321,942
Total loans, net of unamortized deferred loan fees and discounts	331,689		352,584	321,942
Allowance for loan losses	12,789	$ 13,993	15,676	$ 16,929	$ 18,055	$ 16,375
Residential [Member]
Construction:
Total construction	46,067		38,618
Originated [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	1,293,903		1,083,391
Consumer:
Total consumer loans	353,839		357,067
Commercial	172,714		126,611
Construction:
Total construction	83,691		45,680
Total loans, net of deferred loan fees and discounts	1,904,147		1,612,749
Total principal balance of loans owed, net of charge-offs	1,910,379		1,617,542
Unamortized net deferred loan fees	(6,232)		(4,793)
Total loans, net of unamortized deferred loan fees and discounts	1,904,147		1,612,749
Noncovered loans	1,904,147		1,612,749
Total loans, net of unamortized deferred loan fees and discounts	1,904,147		1,612,749
Allowance for loan losses	30,340		29,860
Originated [Member] | Residential 1-4 family [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	191,626		154,594
Originated [Member] | Home equity loans [Member]
Consumer:
Total consumer loans	32,585		23,559
Originated [Member] | Auto Indirect [Member]
Consumer:
Total consumer loans			112
Originated [Member] | Other [Member]
Consumer:
Total consumer loans	29,447		28,230
Originated [Member] | Commercial [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	1,102,277		928,797
Construction:
Total construction	53,279		24,545
Originated [Member] | Home equity lines of credit [Member]
Consumer:
Total consumer loans	291,807		305,166
Originated [Member] | Residential [Member]
Construction:
Total construction	30,412		21,135
PNCI [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	434,209		497,046
Consumer:
Total consumer loans	38,992		50,152
Commercial	21,623		40,899
Construction:
Total construction	25,652		28,781
Total loans, net of deferred loan fees and discounts	520,476		616,878
Total principal balance of loans owed, net of charge-offs	534,973		634,490
Discounts to principal balance of loans owed, net of charge-offs	(14,497)		(17,612)
Total loans, net of unamortized deferred loan fees and discounts	520,476		616,878
Noncovered loans	520,476		616,878
Total loans, net of unamortized deferred loan fees and discounts	520,476		616,878
Allowance for loan losses	2,339		3,296
PNCI [Member] | Residential 1-4 family [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	108,267		120,821
PNCI [Member] | Home equity loans [Member]
Consumer:
Total consumer loans	4,161		6,985
PNCI [Member] | Other [Member]
Consumer:
Total consumer loans	3,516		4,770
PNCI [Member] | Commercial [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	325,942		376,225
Construction:
Total construction	10,727		11,973
PNCI [Member] | Home equity lines of credit [Member]
Consumer:
Total consumer loans	31,315		38,397
PNCI [Member] | Residential [Member]
Construction:
Total construction	14,925		16,808
PCI - Cash basis [Member]
Consumer:
Total consumer loans	5,336		5,603
Commercial	3		8
Construction:
Total loans, net of deferred loan fees and discounts	5,339		5,611
Total principal balance of loans owed, net of charge-offs	13,249		14,805
Discounts to principal balance of loans owed, net of charge-offs	(7,910)		(9,194)
Total loans, net of unamortized deferred loan fees and discounts	5,339		5,611
Noncovered loans	5,339		5,611
Total loans, net of unamortized deferred loan fees and discounts	5,339		5,611
Allowance for loan losses	310		348
PCI - Cash basis [Member] | Home equity loans [Member]
Consumer:
Total consumer loans	124		125
PCI - Cash basis [Member] | Home equity lines of credit [Member]
Consumer:
Total consumer loans	5,212		5,478
PCI - Other [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	28,970		34,922
Consumer:
Total consumer loans	4,914		4,262
Commercial	4,990		7,427
Construction:
Total construction	730		675
Total loans, net of deferred loan fees and discounts	39,604		47,286
Total principal balance of loans owed, net of charge-offs	45,503		56,016
Discounts to principal balance of loans owed, net of charge-offs	(5,899)		(8,730)
Total loans, net of unamortized deferred loan fees and discounts	39,604		47,286
Noncovered loans	33,718		25,018
Covered loans	5,886		22,268
Total loans, net of unamortized deferred loan fees and discounts	39,604		47,286
Allowance for loan losses	3,529		3,081
PCI - Other [Member] | Residential 1-4 family [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	2,212		4,005
PCI - Other [Member] | Home equity loans [Member]
Consumer:
Total consumer loans	1,495		645
PCI - Other [Member] | Other [Member]
Consumer:
Total consumer loans	64		74
PCI - Other [Member] | Commercial [Member]
Mortgage loans on real estate:
Total mortgage loan on real estate	26,758		30,917
PCI - Other [Member] | Home equity lines of credit [Member]
Consumer:
Total consumer loans	3,355		3,543
PCI - Other [Member] | Residential [Member]
Construction:
Total construction	$ 730		$ 675