Rollforward of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 100,302	$ 93,323	$ 100,604	$ 91,649
Charge-offs	(10,277)	(12,489)	(22,037)	(21,193)
Recoveries	1,273	2,463	2,731	3,841
Provision	7,000	14,500	17,000	23,500
Ending balance	98,298	97,797	98,298	97,797
Ending balance: individually evaluated for impairment	4,461	8,436	4,461	8,436	$ 6,683
Ending balance: collectively evaluated for impairment	93,837	89,361	93,837	89,361
Ending balance: loans	11,631,628	10,848,578	11,631,628	10,848,578
Ending balance: individually evaluated for impairment	52,347	77,445	52,347	77,445
Ending balance: collectively evaluated for impairment	11,579,281	10,771,133	11,579,281	10,771,133
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	81,057	73,197	81,156	71,657
Charge-offs	(6,616)	(9,602)	(13,934)	(15,583)
Recoveries	479	1,631	950	2,329
Provision	2,171	11,632	8,919	18,455
Ending balance	77,091	76,858	77,091	76,858
Ending balance: individually evaluated for impairment	4,340	8,362	4,340	8,362
Ending balance: collectively evaluated for impairment	72,751	68,496	72,751	68,496
Ending balance: loans	5,441,779	5,208,864	5,441,779	5,208,864
Ending balance: individually evaluated for impairment	41,556	68,359	41,556	68,359
Ending balance: collectively evaluated for impairment	5,400,223	5,140,505	5,400,223	5,140,505
Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	9,738	10,512	9,312	10,569
Charge-offs	(1,108)	(263)	(2,850)	(449)
Recoveries	73	358	303	419
Provision	3,523	1,298	5,461	1,366
Ending balance	12,226	11,905	12,226	11,905
Ending balance: individually evaluated for impairment	121	74	121	74
Ending balance: collectively evaluated for impairment	12,105	11,831	12,105	11,831
Ending balance: loans	5,804,252	5,243,881	5,804,252	5,243,881
Ending balance: individually evaluated for impairment	10,791	9,086	10,791	9,086
Ending balance: collectively evaluated for impairment	5,793,461	5,234,795	5,793,461	5,234,795
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	9,460	9,510	10,083	9,311
Charge-offs	(2,553)	(2,624)	(5,253)	(5,161)
Recoveries	721	474	1,478	1,093
Provision	1,312	1,601	2,632	3,718
Ending balance	8,940	8,961	8,940	8,961
Ending balance: collectively evaluated for impairment	8,940	8,961	8,940	8,961
Ending balance: loans	365,599	368,029	365,599	368,029
Ending balance: collectively evaluated for impairment	365,599	368,029	365,599	368,029
Leases [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	47	104	53	112
Provision	(6)	(31)	(12)	(39)
Ending balance	41	73	41	73
Ending balance: collectively evaluated for impairment	41	73	41	73
Ending balance: loans	19,998	27,804	19,998	27,804
Ending balance: collectively evaluated for impairment	$ 19,998	$ 27,804	$ 19,998	$ 27,804