Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses by Loan and Lease Class (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				$ 158,059				$ 142,012	$ 158,059	$ 142,012	$ 125,926
Charge- Offs									(59,585)	(51,514)	(31,068)
Recoveries									15,833	11,809	6,713
Net Charge- Offs									(43,752)	(39,705)	(24,355)
Provision for credit losses	$ 16,699	$ 16,768	$ 16,756	10,850	$ 12,705	$ 14,639	$ 16,640	11,768	61,073	55,752	40,441
Balance at End of Year	175,380				158,059				175,380	158,059	142,012
Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				150,791				135,285	150,791	135,285	117,952
Charge- Offs									(57,883)	(50,466)	(30,174)
Recoveries									11,215	10,550	6,023
Net Charge- Offs									(46,668)	(39,916)	(24,151)
Provision for credit losses									64,559	55,422	41,484
Balance at End of Year	168,682				150,791				168,682	150,791	135,285
Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				7,268				6,727	7,268	6,727	7,974
Charge- Offs									(1,702)	(1,048)	(894)
Recoveries									4,618	1,259	690
Net Charge- Offs									2,916	211	(204)
Provision for credit losses									(3,486)	330	(1,043)
Balance at End of Year	6,698				7,268				6,698	7,268	6,727
Commercial real estate | Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				46,635				41,741	46,635	41,741	37,588
Charge- Offs									(2,178)	(6,657)	(4,443)
Recoveries									2,311	3,669	1,117
Net Charge- Offs									133	(2,988)	(3,326)
Provision for credit losses									3,513	7,882	7,479
Balance at End of Year	50,281				46,635				50,281	46,635	41,741
Commercial real estate | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				4,538					4,538
Balance at End of Year	4,976				4,538				4,976	4,538
Commercial and industrial | Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				47,991				41,023	47,991	41,023	32,645
Charge- Offs									(26,188)	(19,584)	(3,562)
Recoveries									1,275	2,508	1,773
Net Charge- Offs									(24,913)	(17,076)	(1,789)
Provision for credit losses									28,885	24,044	10,167
Balance at End of Year	51,963				47,991				51,963	47,991	41,023
Commercial and industrial | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				500					500
Balance at End of Year	(415)				500				(415)	500
Commercial leases | Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				3,280				2,541	3,280	2,541	2,398
Charge- Offs									(1,017)	(962)	(544)
Recoveries									6	66	101
Net Charge- Offs									(1,011)	(896)	(443)
Provision for credit losses									3,377	1,635	586
Balance at End of Year	5,646				3,280				5,646	3,280	2,541
Other | Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				1,392				1,013	1,392	1,013	759
Charge- Offs									(4,099)	(2,729)	(1,691)
Recoveries									1,255	131	55
Net Charge- Offs									(2,844)	(2,598)	(1,636)
Provision for credit losses									3,295	2,977	1,890
Balance at End of Year	1,843				1,392				1,843	1,392	1,013
Total commercial loans and leases | Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				99,298				86,318	99,298	86,318	73,390
Charge- Offs									(33,482)	(29,932)	(10,240)
Recoveries									4,847	6,374	3,046
Net Charge- Offs									(28,635)	(23,558)	(7,194)
Provision for credit losses									39,070	36,538	20,122
Balance at End of Year	109,733				99,298				109,733	99,298	86,318
Total commercial loans and leases | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				5,038					5,038
Balance at End of Year	4,561				5,038				4,561	5,038
Direct installment | Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				21,391				21,587	21,391	21,587	20,538
Charge- Offs									(12,401)	(10,153)	(10,844)
Recoveries									2,015	1,822	1,527
Net Charge- Offs									(10,386)	(8,331)	(9,317)
Provision for credit losses									9,931	8,135	10,366
Balance at End of Year	20,936				21,391				20,936	21,391	21,587
Direct installment | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				1,005					1,005
Balance at End of Year	1,553				1,005				1,553	1,005
Residential mortgages | Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				10,082				7,909	10,082	7,909	8,024
Charge- Offs									(595)	(441)	(1,010)
Recoveries									184	74	85
Net Charge- Offs									(411)	(367)	(925)
Provision for credit losses									5,836	2,540	810
Balance at End of Year	15,507				10,082				15,507	10,082	7,909
Residential mortgages | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				632					632
Balance at End of Year	484				632				484	632
Indirect installment | Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				10,564				9,889	10,564	9,889	7,504
Charge- Offs									(9,201)	(7,855)	(6,427)
Recoveries									3,708	2,015	1,190
Net Charge- Offs									(5,493)	(5,840)	(5,237)
Provision for credit losses									6,896	6,515	7,622
Balance at End of Year	11,967				10,564				11,967	10,564	9,889
Indirect installment | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				221					221
Balance at End of Year	177				221				177	221
Consumer lines of credit | Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				9,456				9,582	9,456	9,582	8,496
Charge- Offs									(2,204)	(2,085)	(1,653)
Recoveries									461	265	175
Net Charge- Offs									(1,743)	(1,820)	(1,478)
Provision for credit losses									2,826	1,694	2,564
Balance at End of Year	10,539				9,456				10,539	9,456	9,582
Consumer lines of credit | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				372					372
Balance at End of Year	(77)				372				(77)	372
Total consumer loans | Originated Loans and Leases
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				51,493				48,967	51,493	48,967	44,562
Charge- Offs									(24,401)	(20,534)	(19,934)
Recoveries									6,368	4,176	2,977
Net Charge- Offs									(18,033)	(16,358)	(16,957)
Provision for credit losses									25,489	18,884	21,362
Balance at End of Year	58,949				51,493				58,949	51,493	48,967
Total consumer loans | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				2,230					2,230
Balance at End of Year	2,137				2,230				2,137	2,230
Purchased credit-impaired loans | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				572				834	572	834	660
Charge- Offs									(469)	(399)	(64)
Recoveries									36	42	19
Net Charge- Offs									(433)	(357)	(45)
Provision for credit losses									496	95	219
Balance at End of Year	635				572				635	572	834
Other acquired loans | Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year				$ 6,696				$ 5,893	6,696	5,893	7,314
Charge- Offs									(1,233)	(649)	(830)
Recoveries									4,582	1,217	671
Net Charge- Offs									3,349	568	(159)
Provision for credit losses									(3,982)	235	(1,262)
Balance at End of Year	$ 6,063				$ 6,696				$ 6,063	$ 6,696	$ 5,893