Loans and Allowance for Credit Losses - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) Contract loan	Dec. 31, 2015 USD ($) Contract	Dec. 31, 2014 USD ($) Contract	Dec. 31, 2013 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses		$ 18,480,000	$ 14,948,000	$ 11,196,000
Charge-offs			18,868,000	$ 17,472,000
Recorded Investment			$ 50,844,000
Number of loans modified | Contract		65	61	77
Total amount of loan modified	$ 10,275,000	$ 10,275,000	$ 10,956,000	$ 15,967,000
Gain (loss) on sale of impaired loans			444,000
Off balance sheet reserve to nonperforming loans			13,000
Loans held for sale	7,052,000	7,052,000	5,763,000
Total gains or losses included in earnings		589,000	111,000
Loans with modification to rate and payment due to amortization		$ 6,600,000	3,700,000	$ 600,000
Minimum days required for conversion of trouble debt to be in default		90 days
Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded commitments related to nonperforming loans	1,800,000	$ 1,800,000	141,000
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non Accrual Status Of Loans After Number Of Days Past Due		150 days
Gear Manufacturer [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Chargeoffs of loans	500,000
Nonaccrual	3,100,000	$ 3,100,000
Local Energy Company [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Chargeoffs of loans	1,300,000
Reserve for Off-balance Sheet Activities [Member] | Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Off balance sheet reserve to nonperforming loans	0	0
Steel and Aluminum Servicing Company [Member] | Commercial and Industrial Sector [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Chargeoffs of loans		4,200,000
Nonaccrual	7,600,000	$ 7,600,000
Local Energy Company [Member] | Commercial and Industrial Sector [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans modified | loan		2
Total amount of loan modified			$ 2,300,000		$ 1,000,000
Nonaccrual	$ 3,300,000	$ 3,300,000