Rollforward of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 98,298	$ 97,797	$ 100,604	$ 91,649
Charge-offs	(3,964)	(11,871)	(26,001)	(33,064)
Recoveries	1,218	963	3,949	4,804
Provision	5,750	11,500	22,750	35,000
Ending balance	101,302	98,389	101,302	98,389
Ending balance: individually evaluated for impairment	4,610	3,185	4,610	3,185	$ 6,683
Ending balance: collectively evaluated for impairment	96,692	95,204	96,692	95,204
Ending balance: loans	11,964,724	10,997,028	11,964,724	10,997,028
Ending balance: individually evaluated for impairment	46,339	71,182	46,339	71,182
Ending balance: collectively evaluated for impairment	11,918,385	10,925,846	11,918,385	10,925,846
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	77,091	76,858	81,156	71,657
Charge-offs	(1,252)	(9,151)	(15,186)	(24,734)
Recoveries	628	190	1,578	2,519
Provision	4,090	8,166	13,009	26,621
Ending balance	80,557	76,063	80,557	76,063
Ending balance: individually evaluated for impairment	4,454	3,060	4,454	3,060
Ending balance: collectively evaluated for impairment	76,103	73,003	76,103	73,003
Ending balance: loans	5,786,712	5,102,259	5,786,712	5,102,259
Ending balance: individually evaluated for impairment	35,474	62,872	35,474	62,872
Ending balance: collectively evaluated for impairment	5,751,238	5,039,387	5,751,238	5,039,387
Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	12,226	11,905	9,312	10,569
Charge-offs	(462)	(439)	(3,312)	(888)
Recoveries	54	201	357	620
Provision	236	1,844	5,697	3,210
Ending balance	12,054	13,511	12,054	13,511
Ending balance: individually evaluated for impairment	156	125	156	125
Ending balance: collectively evaluated for impairment	11,898	13,386	11,898	13,386
Ending balance: loans	5,797,151	5,447,874	5,797,151	5,447,874
Ending balance: individually evaluated for impairment	10,865	8,310	10,865	8,310
Ending balance: collectively evaluated for impairment	5,786,286	5,439,564	5,786,286	5,439,564
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	8,940	8,961	10,083	9,311
Charge-offs	(2,250)	(2,281)	(7,503)	(7,442)
Recoveries	536	572	2,014	1,665
Provision	1,453	1,495	4,085	5,213
Ending balance	8,679	8,747	8,679	8,747
Ending balance: collectively evaluated for impairment	8,679	8,747	8,679	8,747
Ending balance: loans	375,250	422,450	375,250	422,450
Ending balance: collectively evaluated for impairment	375,250	422,450	375,250	422,450
Leases [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	41	73	53	112
Provision	(29)	(5)	(41)	(44)
Ending balance	12	68	12	68
Ending balance: collectively evaluated for impairment	12	68	12	68
Ending balance: loans	5,611	24,445	5,611	24,445
Ending balance: collectively evaluated for impairment	$ 5,611	$ 24,445	$ 5,611	$ 24,445