Loans and Allowance for Credit Losses - Schedule of Activity in Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Impaired [Line Items]
Balance at beginning of period	$ 51,139	$ 60,407	$ 54,003	$ 40,716
(Charge-offs)/recoveries	749	104	577	303
(Payments)/disbursements	(6,423)	(2,277)	(12,346)	(12,517)
Additions	1,954	3,801	5,185	33,533
Balance at end of period	47,419	62,035	47,419	62,035
Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Balance at beginning of period	9,043	20,064	12,088	16,802
(Charge-offs)/recoveries	(7)	59	(158)	(5)
(Payments)/disbursements	453	(1,488)	(3,525)	(7,604)
Additions	1,954	3,157	3,038	12,599
Balance at end of period	11,443	21,792	11,443	21,792
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Balance at beginning of period	34,927	33,059	34,705	18,327
(Charge-offs)/recoveries	753	6	731	366
(Payments)/disbursements	(6,601)	(361)	(7,570)	(3,418)
Additions			1,213	17,429
Balance at end of period	29,079	32,704	29,079	32,704
Residential [Member]
Financing Receivable, Impaired [Line Items]
Balance at beginning of period	3,771	3,640	3,315	2,985
(Charge-offs)/recoveries			23
(Payments)/disbursements	(75)	(294)	(144)	(576)
Additions			502	937
Balance at end of period	3,696	3,346	3,696	3,346
Consumer Loan [Member]
Financing Receivable, Impaired [Line Items]
Balance at beginning of period	3,398	3,644	3,895	2,602
(Charge-offs)/recoveries	3	39	(19)	(58)
(Payments)/disbursements	(200)	(134)	(1,107)	(919)
Additions		644	432	2,568
Balance at end of period	$ 3,201	$ 4,193	$ 3,201	$ 4,193