Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
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		$ 3,508,000
Unfunded commitments related to nonperforming loans	$ 200,000		$ 1,800,000
Off balance sheet reserve to nonperforming loans	100,000		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Total gains or losses included in earnings	$ 0	0
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	$ 200,000	3,900,000
Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for sale, Amount	3,600,000		$ 0
Loans and Leases Receivable, Gain (Loss) on Sales, Net		$ 0
Non Accrual Loans [Member] | Steel and Aluminum Servicing Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	7,400,000
Non Accrual Loans [Member] | Coal Industry Manufacturer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	3,600,000
Non Accrual Loans [Member] | Industrial Manufacturer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	3,000,000
Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	3,200,000
2012 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	1,000,000
2014 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	$ 2,200,000