Loans and Allowance for Credit Losses - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs	$ 7,109,000	$ 3,646,000
Unfunded commitments related to nonperforming loans	600,000		0
Off balance sheet reserve to nonperforming loans	0
Loans held for sale	3,000,000		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	716,000
Total gains or losses included in earnings	105,000	0
Nonaccrual	33,565,000		42,667,000
Recorded investment	46,195,000		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	300,000	200,000
Loans Held For Sale [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		716,000
Total gains or losses included in earnings		0
Water Facility Construction [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investment	9,300,000
Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	6,100,000
Charge-off recognized	3,300,000
2012 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	2,400,000
2014 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	$ 3,100,000