Loans and Allowance for Credit Losses - Credit Risk Profile by Creditworthiness (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 4,229,752	$ 4,204,704	$ 4,159,531
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	4,006,158	3,916,202
OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	111,560	120,205
Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	112,034	164,775
Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	3,522
Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	223,594	288,502
Commercial Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,012,315	1,019,822
Commercial Loan [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	933,092	925,868
Commercial Loan [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	35,231	31,049
Commercial Loan [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	43,992	62,905
Commercial Loan [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial Loan [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	79,223	93,954
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	66,243	87,438
Construction Loans [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	52,706	64,353
Construction Loans [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,030	925
Construction Loans [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	12,507	18,638
Construction Loans [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	3,522
Construction Loans [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	13,537	23,085
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,269,830	1,241,565
Residential Portfolio Segment [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,253,021	1,224,849
Residential Portfolio Segment [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	4,280	5,647
Residential Portfolio Segment [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	12,529	11,069
Residential Portfolio Segment [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Residential Portfolio Segment [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	16,809	16,716
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,280,784	1,273,661
Commercial Real Estate Portfolio Segment [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,166,928	1,119,093
Commercial Real Estate Portfolio Segment [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	71,017	82,581
Commercial Real Estate Portfolio Segment [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	42,839	71,987
Commercial Real Estate Portfolio Segment [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial Real Estate Portfolio Segment [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	113,856	154,568
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	600,580	582,218
Consumer Loan [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	600,411	582,039
Consumer Loan [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2	3
Consumer Loan [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	167	176
Consumer Loan [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Consumer Loan [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 169	$ 179