Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended
	Apr. 26, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Apr. 24, 2012	Oct. 26, 2011
Summary Of Significant Accounting Policies [Line Items]
Stock split conversion ratio	Three-for-two
Percentage of stock dividend declared	50.00%
Common stock, shares authorized		80,000,000	80,000,000		80,000,000
Stock repurchase program, number of shares authorized to be repurchased						750,000
Loans past due		90 days
Impairment recorded		$ 0	$ 0	$ 0
Other identifiable intangible assets, amortized period (years)		7 years
Goodwill and intangible assets with indefinite lives		71,865,000	71,865,000
Carrying amount of goodwill arising from acquisitions, is deductible for federal income tax purposes		49,609,000	49,609,000
Securities sold under agreements to repurchase maturity range (in days)		one to four
Interest-bearing time deposits, maturity period (in days)		90 days or less
Unrealized gains in available-for-sale securities, included in accumulated other comprehensive income		59,669,000	54,568,000
Minimum pension liability, included in accumulated other comprehensive income		8,360,000	7,343,000
Recorded stock option expense		$ 334,000	$ 427,000	$ 387,000