Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Allowance for Doubtful Accounts Receivable [Roll Forward]
Provision for credit losses	$ 6,903	$ 3,229
Ending Balance			$ 53,732		$ 48,676
Ending balance: individually evaluated for impaired			11,473		3,017
Ending balance: collectively evaluated for impaired			42,259		45,659
Total			5,381,305	$ 5,407,376	4,907,961
Ending balance: individually evaluated for impaired			50,491		38,366
Ending balance: collectively evaluated for impaired			5,330,814		4,869,595
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance			3,154		2,445
Total			1,434,623		1,217,398
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance			18,496		6,582
Ending balance: individually evaluated for impaired			2,141		376
Ending balance: collectively evaluated for impaired			16,355		6,206
Total			2,027,072		1,761,101
Ending balance: individually evaluated for impaired			10,360		5,819
Ending balance: collectively evaluated for impaired			2,016,712		1,755,282
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance			27,552		34,701
Ending balance: individually evaluated for impaired			9,045		2,466
Ending balance: collectively evaluated for impaired			18,507		32,235
Total			1,131,594		1,148,460
Ending balance: individually evaluated for impaired			33,278		26,260
Ending balance: collectively evaluated for impaired			1,098,316		1,122,200
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance			1,114		614
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			1,114		614
Total			246,961		240,122
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			246,961		240,122
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending balance: individually evaluated for impaired			287		175
Ending balance: collectively evaluated for impaired			2,867		2,270
Ending balance: individually evaluated for impaired			6,853		6,287
Ending balance: collectively evaluated for impaired			1,427,770		1,211,111
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance			3,416		4,334
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			3,416		4,334
Total			541,055		540,880
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			541,055		540,880
Acquired Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	46	19
Charge-offs	(20)	(15)
Recoveries	37	7
Provision for credit losses	(28)	20
Ending Balance	46	19	35	46	31
Total			384,190	400,015
Acquired Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	6	19
Charge-offs	(16)	(8)
Recoveries	17	6
Provision for credit losses	6	14
Ending Balance	6	19	13	6	31
Total			213,634	220,251
Acquired Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	29	0
Charge-offs	0	0
Recoveries	0	0
Provision for credit losses	(27)	0
Ending Balance	29	0	2	29	0
Total			125,463	126,911
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	11	0
Charge-offs	0	0
Recoveries	7	0
Provision for credit losses	2	0
Ending Balance	11	0	20	11	0
Total			38,402	40,642
Acquired Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0	0
Charge-offs	0	0
Recoveries	6	0
Provision for credit losses	(6)	0
Ending Balance	0	0	0	0	0
Total			1,120	5,963
Acquired Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0	0
Charge-offs	(4)	(7)
Recoveries	7	1
Provision for credit losses	(3)	6
Ending Balance	0	0	0	0	0
Total			5,571	6,248
Originated Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	48,252	50,166
Charge-offs	(2,082)	(5,519)
Recoveries	596	789
Provision for credit losses	6,931	3,209
Ending Balance	48,252	50,166	53,697	48,252	48,645
Total			4,997,115	5,007,361
Originated Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	2,753	2,492
Charge-offs	(455)	(465)
Recoveries	75	122
Provision for credit losses	768	265
Ending Balance	2,753	2,492	3,141	2,753	2,414
Total			1,220,989	1,206,119
Originated Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	17,328	6,619
Charge-offs	(168)	(31)
Recoveries	69	117
Provision for credit losses	1,265	(123)
Ending Balance	17,328	6,619	18,494	17,328	6,582
Total			1,901,609	1,892,185
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	23,418	35,974
Charge-offs	(290)	(3,825)
Recoveries	256	368
Provision for credit losses	4,148	2,184
Ending Balance	23,418	35,974	27,532	23,418	34,701
Total			1,093,192	1,122,741
Originated Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	1,349	577
Charge-offs	0	0
Recoveries	1	54
Provision for credit losses	(236)	(17)
Ending Balance	1,349	577	1,114	1,349	614
Total			245,841	242,905
Originated Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	3,404	4,504
Charge-offs	(1,169)	(1,198)
Recoveries	195	128
Provision for credit losses	986	900
Ending Balance	$ 3,404	$ 4,504	3,416	3,404	$ 4,334
Total			$ 535,484	$ 543,411