Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended
	Apr. 22, 2014	Dec. 31, 2014 Location Bank	Dec. 31, 2013	Dec. 31, 2012	Oct. 28, 2014	May 31, 2013	Apr. 24, 2012
Summary Of Significant Accounting Policies [Line Items]
Number of bank		1
Number of location		62
Stock repurchase program, number of shares authorized to be repurchased					1,500,000
Stock repurchase program expiration date		Sep. 30, 2017
Minimum number of shares that company is required to repurchase		0
Stock repurchased under authorization		0	0	0
Stock split conversion ratio		Two-for-one
Percentage of stock dividend declared	100.00%
Percentage of common stock acquired						100.00%
Consolidation of bank charters description		Eleven bank charters into one charter
Common stock, shares authorized		80,000,000	80,000,000				80,000,000
Loans past due		90 days
Carrying amount of purchased credit impaired loans		$ 20,194,000	$ 27,926,000
Contractual balance		3,275,000	3,970,000
Impairment recorded		0	0	0
Other identifiable intangible assets, amortized period (years)		7 years
Goodwill and intangible assets with indefinite lives		94,882,000	94,882,000
Carrying amount of goodwill arising from acquisitions, is deductible for federal income tax purposes		72,626,000	72,626,000
Other intangible assets mortgage servicing rights		1,834,000	1,684,000
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		50,162,000	14,688,000
Minimum pension liability, included in accumulated other comprehensive income		2,673,000	4,324,000
Recorded stock option expense		709,000	411,000	334,000
Purchased Credit Impaired Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Carrying amount of purchased credit impaired loans		$ 2,151,000	$ 2,707,000