Summary of Significant Accounting Policies - Additional Information (Detail)			1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 28, 2020 USD ($) shares	Apr. 23, 2019 shares	Jul. 28, 2020 shares	Jun. 30, 2020 USD ($) shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2020 USD ($) Location shares	Jun. 30, 2019 USD ($) shares	Dec. 31, 2019 USD ($) shares	Mar. 12, 2020 shares	Jan. 01, 2020	Sep. 02, 2017 shares
Equity, Class of Treasury Stock [Line Items]
Number of locations | Location						78
Stockholders' Equity Note, Stock Split		two-for-one stock split
Common stock, shares authorized | shares		200,000,000		200,000,000	200,000,000	200,000,000	200,000,000	200,000,000
Minimum number of shares that company is required to repurchase | shares						0
Minimum number of days to consider the loans as non-accrual						90 days
Loans individually evaluated for impairment				$ 39,320,000	$ 26,408,000	$ 39,320,000	$ 26,408,000	$ 24,582,000
Contractual balance				53,588,000	35,235,000	53,588,000	35,235,000	33,949,000
Impairment recorded						$ 0	0
Other identifiable intangible assets, amortized period (years)						7 years
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				151,236,000	60,571,000	$ 151,236,000	60,571,000
Minimum pension liability, included in accumulated other comprehensive income				0	1,324,000	0	1,324,000
Stock compensation expense				$ 349,000	313,000	$ 689,000	625,000
First financial antidilutive securities excluded from eps computation | shares				448,000		35,000
Subsequent Event [Member]
Equity, Class of Treasury Stock [Line Items]
Treasury Stock, Shares, Acquired | shares	324,802		0
Treasury Stock, Value, Acquired	$ 8,008,000
Purchased Credit Impaired Loans [Member]
Equity, Class of Treasury Stock [Line Items]
Loans individually evaluated for impairment				$ 7,275,000	464,000	$ 7,275,000	464,000	251,000
Contractual balance				9,818,000	750,000	9,818,000	750,000
Officers [Member]
Equity, Class of Treasury Stock [Line Items]
Restricted stock expense				$ 482,000	$ 345,000	$ 932,000	$ 685,000	$ 345,000
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									4,000,000		2,000,000