Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 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	$ 6,138	$ 4,952	$ 9,646	$ 12,061
Unfunded commitments related to nonperforming loans	2,200		2,200		$ 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	48,076		48,076		36,705
Recorded investment	64,408		$ 64,408		$ 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	300	$ 100	$ 4,200	$ 400
Non Accrual Loans [Member] | Steel and Aluminum Servicing Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	11,400		11,400
Non Accrual Loans [Member] | Coal Industry Manufacturer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	10,500		10,500
Non Accrual Loans [Member] | Oil and Gas Well Services Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	4,400		4,400
Charge-off recognized			2,000
Non Accrual Loans [Member] | Industrial Manufacturer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	3,500		3,500
Charge-off recognized			200
Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	3,700		3,700
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		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	$ 2,400		$ 2,400