Loans and the Allowance for Credit Losses - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015 Contract	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Deferred loan fees and costs	$ 2,200,000	$ 2,400,000
Aggregate amount of residential real estate, home equity and consumer loans classified as substandard	15,500,000	15,200,000
Internally assigned loan grades to residential real estate, home equity and consumer loans	1,500,000	2,200,000
Book value of acquired loans	716,100,000
Fair value of acquired loans	700,800,000
Loans acquired accretable	2,400,000
Book value, acquired loans with deteriorated credit quality	16,000,000
Fair value of acquired loans with deteriorated credit quality	9,700,000
Deteriorated credit quality loans acquired non-accretable difference	3,900,000
Balance loans acquired with deteriorated credit quality	9,300,000
Certain loans acquired in transfer not accounted for as debt securities acquired during period at acquisition non accrual	3,600,000
Certain loans acquired in transfer not accounted for as debt securities acquired during period at acquisition accrual amount	5,700,000
Non-accrual status period	90 days
Number of restructured contracts greater than $1 million	3
Accruing and non accrual TDR permitted interest-only payment period	3 months
Percentage of TDRs defaulted during the period that were restructured within the last twelve months	0.30%
Other real estate owned	5,886,000	4,920,000
Performing Acquired Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Fair value of acquired loans	691,100,000
Loans acquired accretable	9,000,000
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Other real estate owned	800,000	600,000
Foreclosure proceedings in process on residential real estate loans	$ 4,300,000