Rollforward of Allowance for Loan Losses by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Commercial	Sep. 30, 2011 Commercial	Sep. 30, 2012 Commercial	Sep. 30, 2011 Commercial	Sep. 30, 2012 Real estate	Sep. 30, 2011 Real estate	Sep. 30, 2012 Real estate	Sep. 30, 2011 Real estate	Sep. 30, 2012 Consumer	Sep. 30, 2011 Consumer	Sep. 30, 2012 Consumer	Sep. 30, 2011 Consumer	Sep. 30, 2012 Leases	Sep. 30, 2012 Leases	Sep. 30, 2011 Leases	Dec. 31, 2011 Leases	Jun. 30, 2011 Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 72,652	$ 72,442	$ 72,017	$ 73,952		$ 37,942	$ 35,604	$ 37,927	$ 39,138	$ 22,660	$ 22,886	$ 20,486	$ 18,557	$ 12,001	$ 13,941	$ 13,593	$ 16,243	$ 49	$ 11	$ 14		$ 11
Charge-offs	(6,550)	(4,995)	(16,783)	(21,849)		(3,147)	(1,372)	(6,385)	(9,456)	(316)	(48)	(724)	(505)	(3,087)	(3,575)	(9,674)	(11,888)
Recoveries	766	929	2,634	3,573		151	108	401	484	16	9	25	24	599	812	2,208	3,065
Provision	4,500	4,500	13,500	17,200		8,167	3,226	11,170	7,400	(5,591)	(1,033)	(3,018)	3,738	1,916	2,307	5,302	6,065	8	46	(3)
Ending Balance	71,368	72,876	71,368	72,876		43,113	37,566	43,113	37,566	16,769	21,814	16,769	21,814	11,429	13,485	11,429	13,485	57	57	11		11
Ending Balance: individually evaluated for impairment, Allowance	2,658	2,213	2,658	2,213		1,591	1,597	1,591	1,597	1,067	616	1,067	616
Ending Balance: collectively evaluated for impairment, Allowance	68,710	70,663	68,710	70,663		41,522	35,969	41,522	35,969	15,702	21,198	15,702	21,198	11,429	13,485	11,429	13,485	57	57	11
Ending Balance: loans acquired with deteriorated credit quality, Allowance
Ending Balance: loans	5,389,763	4,776,071	5,389,763	4,776,071	4,960,343	2,800,529	2,202,500	2,800,529	2,202,500	2,183,960	2,169,750	2,183,960	2,169,750	386,490	399,526	386,490	399,526	18,784	18,784	4,295	3,834
Ending Balance: individually evaluated for impairment, Loans	32,821	15,867	32,821	15,867		18,380	5,115	18,380	5,115	14,396	10,729	14,396	10,729	45	23	45	23
Ending Balance: collectively evaluated for impairment, Loans	5,356,942	4,760,204	5,356,942	4,760,204		2,782,149	2,197,385	2,782,149	2,197,385	2,169,564	2,159,021	2,169,564	2,159,021	386,445	399,503	386,445	399,503	18,784	18,784	4,295
Ending Balance: loans acquired with deteriorated credit quality, Loans