Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Provision for credit losses	$ 1,214	$ 3,408	$ 2,834	$ 20,306
Ending Balance		54,734		54,734	$ 48,176
Ending balance: individually evaluated for impaired		8,175		8,175	2,181
Ending balance: collectively evaluated for impaired		46,559		46,559	45,995
Total		4,860,652		4,860,652	5,375,847	$ 4,879,347
Ending balance: individually evaluated for impaired		46,252		46,252	31,326
Ending balance: collectively evaluated for impaired		4,814,400		4,814,400	5,344,521
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance		2,367		2,367	2,898
Total		1,185,759		1,185,759	1,423,422
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance		5,584		5,584	17,367
Ending balance: individually evaluated for impaired		430		430	261
Ending balance: collectively evaluated for impaired		5,154		5,154	17,106
Total		1,683,015		1,683,015	1,990,264
Ending balance: individually evaluated for impaired		5,081		5,081	8,189
Ending balance: collectively evaluated for impaired		1,677,934		1,677,934	1,982,075
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance		41,923		41,923	23,144
Ending balance: individually evaluated for impaired		7,739		7,739	1,818
Ending balance: collectively evaluated for impaired		34,184		34,184	21,326
Total		1,207,447		1,207,447	1,154,225
Ending balance: individually evaluated for impaired		35,501		35,501	15,995
Ending balance: collectively evaluated for impaired		1,171,946		1,171,946	1,138,230
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance		476		476	1,341
Ending balance: individually evaluated for impaired		0		0	0
Ending balance: collectively evaluated for impaired		476		476	1,341
Total		229,375		229,375	259,129
Ending balance: individually evaluated for impaired		0		0	0
Ending balance: collectively evaluated for impaired		229,375		229,375	259,129
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending balance: individually evaluated for impaired		6		6	102
Ending balance: collectively evaluated for impaired		2,361		2,361	2,796
Ending balance: individually evaluated for impaired		5,670		5,670	7,142
Ending balance: collectively evaluated for impaired		1,180,089		1,180,089	1,416,280
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance		4,384		4,384	3,426
Ending balance: individually evaluated for impaired		0		0	0
Ending balance: collectively evaluated for impaired		4,384		4,384	3,426
Total		555,056		555,056	548,807
Ending balance: individually evaluated for impaired		0		0	0
Ending balance: collectively evaluated for impaired		555,056		555,056	548,807
Acquired Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	172		19
Charge-offs	(26)		(43)
Recoveries	28		106
Provision for credit losses	356		448
Ending Balance	172		19		530	19
Total					424,998	100,876
Acquired Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4		19
Charge-offs	(17)		(26)
Recoveries	18		45
Provision for credit losses	1		(32)
Ending Balance	4		19		6	19
Total					232,560	63,341
Acquired Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	50		0
Charge-offs	0		0
Recoveries	0		4
Provision for credit losses	(6)		40
Ending Balance	50		0		44	0
Total					132,701	25,167
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	118		0
Charge-offs	0		0
Recoveries	1		1
Provision for credit losses	361		479
Ending Balance	118		0		480	0
Total					42,893	8,399
Acquired Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0		0
Charge-offs	0		0
Recoveries	4		5
Provision for credit losses	(4)		(5)
Ending Balance	0		0		0	0
Total					10,222	1,781
Acquired Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0		0
Charge-offs	(9)		(17)
Recoveries	5		51
Provision for credit losses	4		(34)
Ending Balance	0		0		0	0
Total					6,622	2,188
Originated Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	47,895	59,821	50,166	50,812
Charge-offs	(1,893)	(8,807)	(10,010)	(18,453)
Recoveries	786	312	5,104	2,069
Provision for credit losses	858	3,408	2,386	20,306
Ending Balance	47,895	59,821	50,166	50,812	47,646	50,166
Total					4,950,849	4,778,471
Originated Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	2,116	2,605	2,492	2,606
Charge-offs	(344)	(374)	(954)	(976)
Recoveries	67	147	259	407
Provision for credit losses	1,053	(11)	1,095	330
Ending Balance	2,116	2,605	2,492	2,606	2,892	2,492
Total					1,190,862	1,165,851
Originated Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	7,307	5,862	6,619	11,924
Charge-offs	(35)	(10)	(95)	(418)
Recoveries	60	20	206	803
Provision for credit losses	9,991	(288)	10,593	(6,725)
Ending Balance	7,307	5,862	6,619	11,924	17,323	6,619
Total					1,857,563	1,717,043
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	33,372	46,357	35,974	31,035
Charge-offs	(499)	(7,163)	(5,776)	(13,308)
Recoveries	183	63	3,819	261
Provision for credit losses	(10,392)	2,666	(11,353)	23,935
Ending Balance	33,372	46,357	35,974	31,035	22,664	35,974
Total					1,111,332	1,131,148
Originated Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	768	480	577	887
Charge-offs	0	0	0	0
Recoveries	369	0	465	227
Provision for credit losses	204	(4)	299	(638)
Ending Balance	768	480	577	887	1,341	577
Total					248,907	217,840
Originated Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,332	4,517	4,504	4,360
Charge-offs	(1,015)	(1,260)	(3,185)	(3,751)
Recoveries	107	82	355	371
Provision for credit losses	2	1,045	1,752	3,404
Ending Balance	$ 4,332	$ 4,517	$ 4,504	$ 4,360	3,426	4,504
Total					$ 542,185	$ 546,589