Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 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	$ 8,807	$ 2,401	$ 18,453	$ 14,462
Unfunded commitments related to nonperforming loans	400		400		$ 100
Off balance sheet reserve to nonperforming loans	0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Total gains or losses included in earnings	0	0	0	105
Nonaccrual	40,540		40,540		36,705
Recorded investment	54,826		$ 54,826		$ 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	100	$ 0	$ 4,300	$ 400
Non Accrual Loans [Member] | Steel and Aluminum Servicing Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	11,800		11,800
Non Accrual Loans [Member] | Coal Industry Manufacturer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	4,000		4,000
Charge-off recognized			6,500
Non Accrual Loans [Member] | Oil and Gas Well Services Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	4,300		4,300
Charge-off recognized			2,000
Non Accrual Loans [Member] | Industrial Manufacturer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	3,300		3,300
Charge-off recognized			400
Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	3,400		3,400
2012 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	1,000		1,000
Charge-off recognized			1,300
2014 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	$ 2,300		$ 2,300