Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Location shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Jan. 01, 2018	Sep. 02, 2017 shares	Jun. 25, 2017 shares
Equity, Class of Treasury Stock [Line Items]
Number of locations | Location	73
Stock repurchase program expiration date	Sep. 30, 2020
Minimum number of shares that company is required to repurchase | shares	0
Stock repurchased under authorization | shares	0	0	0
Minimum number of days to consider the loans as non-accrual	90 days
Loans individually evaluated for impairment	$ 27,534,000	$ 17,670,000
Contractual balance	36,614,000	23,851,000
Impairment recorded	$ 0	0	$ 0
Other identifiable intangible assets, amortized period (years)	7 years
Goodwill and intangible assets with indefinite lives	$ 171,565,000	139,971,000
Carrying amount of goodwill arising from acquisitions, is deductible for federal income tax purposes	$ 22,526,000	26,618,000
Other intangible assets mortgage servicing rights		1,795,000
Loss on sale of mortgage servicing rights		(215,000)
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	$ 4,169,000	29,156,000
Minimum pension liability, included in accumulated other comprehensive income	(1,324,000)	(2,370,000)
Stock compensation expense	$ 1,508,000	$ 1,745,000	$ 882,000
Federal statutory tax rate	21.00%	35.00%	35.00%
Purchased Credit Impaired Loans [Member]
Equity, Class of Treasury Stock [Line Items]
Loans individually evaluated for impairment	$ 827,000	$ 618,000
Contractual balance	1,157,000	1,865,000
Accounting Standards Update 2018-02 [Member]
Equity, Class of Treasury Stock [Line Items]
Reclassification of standard tax debit within accumulated other comprehensive income to retained earnings	5,759,000
Director [Member]
Equity, Class of Treasury Stock [Line Items]
Restricted stock expense	560,000	483,000	$ 278,000
Officers [Member]
Equity, Class of Treasury Stock [Line Items]
Restricted stock expense	$ 680,000	$ 562,000	$ 381,000
Commercial Bancshares Inc and Commercial State Bank [Member]
Equity, Class of Treasury Stock [Line Items]
Percentage of common stock acquired				100.00%
Maximum [Member]
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, number of shares authorized to be repurchased | shares					1,500,000	2,000,000