Rollforward of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 103,635	$ 100,604
Charge-offs	(13,744)	(11,760)
Recoveries	1,420	1,458
Provision	12,350	10,000
Ending balance	103,661	100,302
Ending balance: individually evaluated for impairment	5,648	5,113	$ 4,711
Ending balance: collectively evaluated for impairment	98,013	95,189
Ending balance: loans	12,549,732	11,458,794
Ending balance: individually evaluated for impairment	59,039	63,534
Ending balance: collectively evaluated for impairment	12,490,693	11,395,260
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	80,888	81,156
Charge-offs	(11,163)	(7,318)
Recoveries	626	471
Provision	9,981	6,748
Ending balance	80,332	81,057
Ending balance: individually evaluated for impairment	5,547	4,986
Ending balance: collectively evaluated for impairment	74,785	76,071
Ending balance: loans	6,261,244	5,242,307
Ending balance: individually evaluated for impairment	37,493	48,190
Ending balance: collectively evaluated for impairment	6,223,751	5,194,117
Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	13,664	9,312
Charge-offs	(114)	(1,742)
Recoveries	73	230
Provision	767	1,938
Ending balance	14,390	9,738
Ending balance: individually evaluated for impairment	101	127
Ending balance: collectively evaluated for impairment	14,289	9,611
Ending balance: loans	5,928,139	5,764,398
Ending balance: individually evaluated for impairment	21,197	15,344
Ending balance: collectively evaluated for impairment	5,906,942	5,749,054
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	9,071	10,083
Charge-offs	(2,467)	(2,700)
Recoveries	721	757
Provision	1,603	1,320
Ending balance	8,928	9,460
Ending balance: collectively evaluated for impairment	8,928	9,460
Ending balance: loans	355,538	428,938
Ending balance: individually evaluated for impairment	349
Ending balance: collectively evaluated for impairment	355,189	428,938
Leases [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	12	53
Provision	(1)	(6)
Ending balance	11	47
Ending balance: collectively evaluated for impairment	11	47
Ending balance: loans	4,811	23,151
Ending balance: collectively evaluated for impairment	$ 4,811	$ 23,151