Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
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	$ 3,508	$ 7,109
Unfunded commitments related to nonperforming loans	2,500		$ 100
Off balance sheet reserve to nonperforming loans	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Total gains or losses included in earnings	0	105
Nonaccrual	46,836		36,705
Recorded investment	$ 61,815		$ 50,844
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	$ 3,900	$ 300
Non Accrual Loans [Member] | Steel and Aluminum Servicing Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	11,500
Non Accrual Loans [Member] | Oil and Gas Well Services Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	6,800
Non Accrual Loans [Member] | Sporting Goods Manufacturer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	3,800
Non Accrual Loans [Member] | Industrial Manufacturer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	3,700
Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	3,800
2012 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	1,300
Charge-off recognized	1,100
2014 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	$ 2,500