Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	$ 11,627	$ 14,069
60 - 89 days past due	2,136	5,319
90 days and greater and still accruing	2,648	2,447
Nonaccrual	59,286	94,518
Total past due and nonaccrual	75,697	116,353
Current	4,154,055	4,088,351
Total	4,229,752	4,204,704	4,159,531
Commercial Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	814	991
60 - 89 days past due	34	620
90 days and greater and still accruing	289	288
Nonaccrual	20,170	29,258
Total past due and nonaccrual	21,307	31,157
Current	991,008	988,665
Total	1,012,315	1,019,822
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	0	2
60 - 89 days past due	0	19
90 days and greater and still accruing	0	15
Nonaccrual	3,073	9,778
Total past due and nonaccrual	3,073	9,814
Current	63,170	77,624
Total	66,243	87,438
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	5,058	6,597
60 - 89 days past due	1,028	2,357
90 days and greater and still accruing	1,182	730
Nonaccrual	10,719	9,283
Total past due and nonaccrual	17,987	18,967
Current	1,251,843	1,222,598
Total	1,269,830	1,241,565
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	3,116	3,339
60 - 89 days past due	48	1,389
90 days and greater and still accruing	0	195
Nonaccrual	25,136	46,023
Total past due and nonaccrual	28,300	50,946
Current	1,252,484	1,222,715
Total	1,280,784	1,273,661
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	2,639	3,140
60 - 89 days past due	1,026	934
90 days and greater and still accruing	1,177	1,219
Nonaccrual	188	176
Total past due and nonaccrual	5,030	5,469
Current	595,550	576,749
Total	$ 600,580	$ 582,218