Summary of Significant Accounting Policies - Additional Information (Detail)		12 Months Ended
	Apr. 22, 2014	Dec. 31, 2016 USD ($) Location shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	Jul. 31, 2015	Apr. 28, 2015 shares	Oct. 28, 2014 shares
Summary Of Significant Accounting Policies [Line Items]
Number of locations | Location		69
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
Common stock, shares authorized | shares		120,000,000	120,000,000			120,000,000
Minimum number of days to consider the loans as non-accrual		Greater than 90 days
Loans individually evaluated for impairment		$ 27,371,000	$ 28,601,000
Contractual balance		38,379,000	35,030,000
Impairment recorded		$ 0	0	$ 0
Other identifiable intangible assets, amortized period (years)		7 years
Goodwill and intangible assets with indefinite lives		$ 139,971,000	139,971,000
Carrying amount of goodwill arising from acquisitions, is deductible for federal income tax purposes		74,376,000	74,376,000
Other intangible assets mortgage servicing rights		$ 1,795,000	1,902,000
Securities sold under agreements to repurchase maturity range (in days)		One to four days
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		$ 21,492,000	51,359,000
Minimum pension liability, included in accumulated other comprehensive income		(3,047,000)	(3,964,000)
Stock compensation expense		882,000	644,000	709,000
Restricted stock expense				$ 0
Purchased Credit Impaired Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Loans individually evaluated for impairment		1,256,000	2,178,000
Contractual balance		1,865,000	2,936,000
Director [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted stock expense		278,000	139,000
Officers [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted stock expense		381,000	$ 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%
Goodwill and intangible assets with indefinite lives		43,922,000
FBC Bancshares, Inc. and First Bank, N.A. [Member] | Purchased Credit Impaired Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Contractual balance		$ 1,704,000
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%