Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non Accrual Status Of Loans After Number Of Days Past Due			90 days
Default Status Of TDRs After Number Of Days Past Due			90 days
Charge-offs	$ 2,401	$ 2,880	$ 14,462	$ 14,629
Unfunded commitments related to nonperforming loans	500		500		$ 0
Off balance sheet reserve to nonperforming loans	0		0		0
Loans held for sale	0		0
Impaired Loans Sold			2,400
Gain on impaired loans sold			400
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	3,112
Total gains or losses included in earnings	0	0	105	77
Nonaccrual	28,803		28,803		42,667
Recorded investment	40,827		$ 40,827		$ 55,251
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non Accrual Status Of Loans After Number Of Days Past Due			150 days
Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans with modifications to rate and payment due to reamortization	0	100	$ 400	500
Loans Held For Sale [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0		3,112
Total gains or losses included in earnings		$ 0		$ 77
Non Accrual Loans [Member] | Industrial Manufacturer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	6,100		6,100
Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	5,700		5,700
2012 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	2,300		2,300
Charge-off recognized			3,300
2014 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	$ 2,900		$ 2,900