Rollforward of Allowance for Loan Losses by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 71,426	$ 72,017
Charge-offs	(4,849)	(4,098)
Recoveries	1,304	1,067
Provision	2,000	4,500
Ending Balance	69,881	73,486
Ending Balance: individually evaluated for impairment, allowances	3,515	4,372
Ending Balance: collectively evaluated for impairment, allowances	66,366	69,114
Ending Balance: loans	6,010,681	5,144,766	5,686,749
Ending Balance: individually evaluated for impairment, loans	26,160	23,652
Ending Balance: collectively evaluated for impairment, loans	5,984,521	5,121,114
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	43,390	37,927
Charge-offs	(1,397)	(269)
Recoveries	374	237
Provision	978	(514)
Ending Balance	43,345	37,381
Ending Balance: individually evaluated for impairment, allowances	3,206	3,918
Ending Balance: collectively evaluated for impairment, allowances	40,139	33,463
Ending Balance: loans	3,305,175	2,583,911
Ending Balance: individually evaluated for impairment, loans	15,974	11,571
Ending Balance: collectively evaluated for impairment, loans	3,289,201	2,572,340
Real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	15,506	20,486
Charge-offs	(195)	(339)
Recoveries	9	6
Provision	(374)	3,083
Ending Balance	14,946	23,236
Ending Balance: individually evaluated for impairment, allowances	309	454
Ending Balance: collectively evaluated for impairment, allowances	14,637	22,782
Ending Balance: loans	2,320,388	2,184,881
Ending Balance: individually evaluated for impairment, loans	10,140	12,054
Ending Balance: collectively evaluated for impairment, loans	2,310,248	2,172,827
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	12,470	13,593
Charge-offs	(3,257)	(3,490)
Recoveries	921	824
Provision	1,395	1,921
Ending Balance	11,529	12,848
Ending Balance: individually evaluated for impairment, allowances
Ending Balance: collectively evaluated for impairment, allowances	11,529	12,848
Ending Balance: loans	365,217	368,175
Ending Balance: individually evaluated for impairment, loans	46	27
Ending Balance: collectively evaluated for impairment, loans	365,171	368,148
Leases [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	60	11
Charge-offs
Recoveries
Provision	1	10
Ending Balance	61	21
Ending Balance: individually evaluated for impairment, allowances
Ending Balance: collectively evaluated for impairment, allowances	61	21
Ending Balance: loans	19,901	7,799	19,084
Ending Balance: individually evaluated for impairment, loans
Ending Balance: collectively evaluated for impairment, loans	$ 19,901	$ 7,799