Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
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	$ 4,952,000	$ 8,103,000	$ 12,061,000	$ 11,749,000
Unfunded commitments related to nonperforming loans	600,000		600,000		$ 0
Off balance sheet reserve to nonperforming loans	0		0		0
Loans held for sale	2,400,000		2,400,000		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	(2,396,000)	0	3,112,000
Total gains or losses included in earnings	0	77,000	105,000	77,000
Nonaccrual	30,395,000		30,395,000		42,667,000
Recorded investment	42,671,000		$ 42,671,000		$ 55,251,000
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	100,000	100,000	$ 400,000	300,000
Loans Held For Sale [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		(2,396,000)		3,112,000
Total gains or losses included in earnings		$ 77,000	0	$ 77,000
Water Facility Construction [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investment	7,400,000		7,400,000
Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	5,900,000		5,900,000
Charge-off recognized			2,300,000
2012 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	2,300,000		2,300,000
Charge-off recognized			3,300,000
2014 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	$ 3,000,000		$ 3,000,000