Loans and Allowance for Credit Losses - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses	$ 11,196,000	$ 19,227,000	$ 20,544,000
Charge-offs	17,472,000	35,178,000	16,969,000
Recorded Investment	55,251,000	59,383,000
Nonaccrual	42,667,000	45,888,000
Loans held for sale	2,502,000	0
Total gains or losses included in earnings	77,000	(770,000)
Loans with modification to rate and payment due to amortization	600,000	2,000,000	4,700,000
Minimum days required for conversion of trouble debt to be in default	90 days
Water Treatment Plant [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans		2,800,000
Gas Well Servicing Operation [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans		3,700,000
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans			3,300,000
Loan maturity extension			6 months
Commercial real estate [Member] | Gas Well Servicing Operation [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans		17,000,000
Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase In Commitments Related To Troubled Debt Restructured Loans	3,700,000
Troubled Debt Restructured Loans Decreased	900,000
Decrease in nonperforming loans	(4,100,000)
Decrease In Specific Allowance For Nonperforming Loans	(600,000)
Unfunded commitments related to nonperforming loans	46,000
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Decrease in nonperforming loans	32,200,000
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses		(5,926,000)	1,090,000
Charge-offs		0	0
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non Accrual Status Of Loans After Number Of Days Past Due	150 days
Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount Of Non Accrual Status Loan	22,100,000
Nonaccrual Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount Of Non Accrual Status Loan	3,800,000
Western Pennsylvania Specialty Metal Processor [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount Of Non Accrual Status Loan	8,000,000
Western Pennsylvania Reals Estate Manager [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	9,900,000
Local Energy Company [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	10,000,000
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
Loans Held for Sale [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gains or losses included in earnings	$ 77,000	$ 625,000
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Modification, Subsequent Default, Number of Contracts		1	0