Loans - Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	$ 123,351	$ 107,430	$ 105,577
Recorded Investment With No Allowance	46,893	69,351	43,083
Recorded Investment With Allowance	35,995	18,798	37,314
Total Recorded Investment	82,888	88,149	80,397
Related Allowance	8,630	6,564	8,237
Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance			1,255
Recorded Investment With Allowance			1,069
Total Recorded Investment			1,069
Related Allowance			900
Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	75,350	55,632	56,784
Recorded Investment With No Allowance	25,880	28,932	21,709
Recorded Investment With Allowance	18,218	15,804	19,096
Total Recorded Investment	44,098	44,736	40,805
Related Allowance	5,340	5,356	4,200
Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	45,158	47,077	44,652
Recorded Investment With No Allowance	18,740	36,821	19,010
Recorded Investment With Allowance	17,777	2,243	17,149
Total Recorded Investment	36,517	39,064	36,159
Related Allowance	3,290	1,113	3,137
Commercial Real Estate, Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	1,479	1,551	1,497
Recorded Investment With No Allowance	1,055	1,237	1,100
Recorded Investment With Allowance
Total Recorded Investment	1,055	1,237	1,100
Related Allowance
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	947	2,623	961
Recorded Investment With No Allowance	834	1,826	864
Recorded Investment With Allowance		751
Total Recorded Investment	834	2,577	864
Related Allowance		95
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Contractual Principal Balance	417	547	428
Recorded Investment With No Allowance	384	535	400
Recorded Investment With Allowance
Total Recorded Investment	384	535	400
Related Allowance
Quarter To Date [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	81,644	86,357	89,013
Quarter To Date [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	535		535
Quarter To Date [Member] | Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	42,452	42,034	44,941
Quarter To Date [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	36,338	39,996	41,126
Quarter To Date [Member] | Commercial Real Estate, Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	1,078	1,259	1,122
Quarter To Date [Member] | Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	849	2,524	879
Quarter To Date [Member] | Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	392	544	410
Year To Date [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	81,644	86,357	86,580
Year To Date [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	535		214
Year To Date [Member] | Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	42,452	42,034	42,630
Year To Date [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	36,338	39,996	40,258
Year To Date [Member] | Commercial Real Estate, Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	1,078	1,259	1,392
Year To Date [Member] | Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	849	2,524	1,617
Year To Date [Member] | Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	$ 392	$ 544	$ 469