Loans and the Allowance for Credit Losses - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Contract	Dec. 31, 2015 USD ($)	Sep. 09, 2016 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Deferred loan fees and costs	$ 300,000	$ 1,000,000
Discount on purchased loans from acquisitions	24,100,000	15,700,000
Aggregate amount of residential real estate, home equity and consumer loans classified as substandard	20,600,000	15,800,000
Internally assigned loan grades to residential real estate, home equity and consumer loans	3,400,000	3,100,000
Loans acquired with deteriorated credit quality	12,859,000	9,289,000
Allowance for loan losses	$ 43,674,000	41,710,000		$ 44,654,000	$ 47,368,000
Number of restructured contracts greater than $1 million | Contract	0
Accruing and non accrual TDR permitted interest-only payment period	3 months
Unfunded commitments to debtors for impaired loans	$ 0	200,000
Other real estate owned	$ 8,206,000	5,669,000
Maximum [Member]
Financing Receivable, Recorded Investment [Line Items]
Percentage of TDRs defaulted during the period that were restructured within the last twelve months	0.10%
Your Community Bankshares, Inc [Member]
Financing Receivable, Recorded Investment [Line Items]
Discount on purchased loans from acquisitions	$ 11,000,000
Book value of acquired loans			$ 1,027,200,000
Fair value of acquired loans			1,013,566,000
Loans acquired accretable			800,000
Pre-acquisition book value, acquired loans with deteriorated credit quality			11,100,000
Contractually required payments, acquired loans with deteriorated credit quality			13,300,000
Fair value of acquired loans with deteriorated credit quality			7,100,000
Deteriorated credit quality loans acquired non-accretable difference			5,300,000
Loans acquired with deteriorated credit quality	5,700,000
Loans acquired with deteriorated credit quality, outstanding customer balance	9,200,000
Allowance for loan losses	0
Your Community Bankshares, Inc [Member] | Acquired Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Fair value of acquired loans			1,006,900,000
Loans acquired accretable			9,200,000
Your Community Bankshares, Inc [Member] | Cost Recovery Method [Member]
Financing Receivable, Recorded Investment [Line Items]
Fair value of acquired loans			$ 2,700,000
ESB Financial Corporation [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans acquired accretable	900,000
Loans acquired with deteriorated credit quality	7,200,000	9,300,000
Allowance for loan losses	1,800,000	0
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans acquired with deteriorated credit quality	943,000	161,000
Allowance for loan losses	4,106,000	4,582,000		$ 5,382,000	$ 5,673,000
Other real estate owned	1,600,000	$ 2,000,000
Foreclosure proceedings in process on residential real estate loans	4,100,000
Residential Real Estate [Member] | Your Community Bankshares, Inc [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate owned from acquisition	$ 3,100,000