Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 30, 2020 USD ($) shares	Mar. 31, 2020 USD ($) shares	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2021 USD ($) Location shares	Sep. 30, 2020 USD ($) shares	Dec. 31, 2020 USD ($)	Jul. 27, 2021 shares	Mar. 27, 2020 USD ($)	Mar. 12, 2020 shares	Jan. 01, 2020
Equity, Class of Treasury Stock [Line Items]
Number of locations | Location					78
Minimum number of shares that company is required to repurchase | shares					0
Treasury Stock, Value, Acquired	$ 8,008,000	$ 8,008,000
Minimum number of days to consider the loans as non-accrual					90 days
Impairment recorded			$ 0	$ 0	$ 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			109,447,000	152,063,000	$ 109,447,000	152,063,000	$ 170,395,000
Retained earnings net of tax after transitional charges			$ 947,724,000	$ 797,202,000	$ 947,724,000	$ 797,202,000	836,729,000		$ 466,000
First financial antidilutive securities excluded from eps computation | shares			127,000	400,000	46,000	15,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased under stock repurchase authorization, shares | shares	(324,802)	(324,802)				(324,802)
Allowance for Credit Losses [Member]
Equity, Class of Treasury Stock [Line Items]
Increase (decrease) in valuation and qualifying accounts and reserves from adjustment							(619,000)
Reserve for Unfunded Commitments [Member]
Equity, Class of Treasury Stock [Line Items]
Increase (decrease) in valuation and qualifying accounts and reserves from adjustment							1,208,000
Cumulative Effect, Period of Adoption, Adjustment [Member]
Equity, Class of Treasury Stock [Line Items]
Retained earnings net of tax after transitional charges									$ 589,000
Commitments to Extend Credit [Member]
Equity, Class of Treasury Stock [Line Items]
Reserve for losses on unfunded commitments			$ 6,751,000	$ 2,309,000	$ 6,751,000	$ 2,309,000	5,486,000
Held-to-maturity Securities [Member]
Equity, Class of Treasury Stock [Line Items]
Securities held to maturity			$ 0	$ 0	$ 0	$ 0	$ 0
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								5,000,000		4,000,000