Summary of Significant Accounting Policies - Additional Information (Detail)				12 Months Ended
	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	Apr. 22, 2014	Dec. 31, 2015 USD ($) Location shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($) shares	Jul. 31, 2015	Apr. 28, 2015 shares	Oct. 28, 2014 shares	May. 31, 2013
Summary Of Significant Accounting Policies [Line Items]
Number of locations | Location				70
Stock repurchase program expiration date				Sep. 30, 2017
Minimum number of shares that company is required to repurchase | shares				0
Stock repurchased under authorization | shares				0	0	0
Stock split conversion ratio				Two-for-one stock split
Percentage of stock dividend declared			100.00%
Dividend declared date				Apr. 22, 2014
Dividend record date				May 15, 2014
Dividend distribution date				Jun. 02, 2014
Percentage of common stock acquired										100.00%
Common stock, shares authorized | shares	120,000,000	120,000,000		120,000,000	120,000,000			120,000,000
Minimum number of days to consider the loans as non-accrual				90 days
Carrying amount of purchased credit impaired loans	$ 2,178,000	$ 2,151,000
Contractual balance	2,936,000	3,275,000		$ 2,936,000	$ 3,275,000
Impairment recorded				$ 0	0	$ 0
Other identifiable intangible assets, amortized period (years)				7 years
Goodwill and intangible assets with indefinite lives	139,971,000	94,882,000		$ 139,971,000	94,882,000
Carrying amount of goodwill arising from acquisitions, is deductible for federal income tax purposes	74,376,000	72,626,000		74,376,000	72,626,000
Other intangible assets mortgage servicing rights	1,902,000	1,834,000		$ 1,902,000	1,834,000
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 net gains in available-for-sale securities, included in accumulated other comprehensive income	51,359,000	50,162,000		$ 51,359,000	50,162,000
Minimum pension liability, included in accumulated other comprehensive income	3,964,000	$ 2,673,000		3,964,000	2,673,000
Recorded stock option expense				644,000	709,000	411,000
Restricted stock expense					$ 0	$ 0
Director [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted stock expense				139,000
Officers [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted stock expense				62,000
FBC Bancshares, Inc. and First Bank, N.A. [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of common stock acquired							100.00%
Contractual balance	1,704,000			1,704,000
Goodwill and intangible assets with indefinite lives	$ 43,922,000			$ 43,922,000
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Stock repurchase program, number of shares authorized to be repurchased | shares									1,500,000
Percentage of stock dividend declared				53.00%