Summary of Significant Accounting Policies - Additional Information (Detail)		7 Months Ended		12 Months Ended
	Apr. 23, 2019 shares	Feb. 22, 2022 shares	Jul. 27, 2021 USD ($) shares	Dec. 31, 2021 USD ($) Location shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Sep. 30, 2021 shares	Sep. 30, 2020 shares	Mar. 27, 2020 USD ($)	Jan. 01, 2020
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
Minimum number of shares that company is required to repurchase | shares				0
Retained earnings net of tax after transitional charges				$ 981,675,000	$ 836,729,000				$ 466,000,000
Minimum number of days to consider the loans as non-accrual				90 days
Amount of allowance for credit loss				$ 0	0
Other identifiable intangible assets, amortized period (years)				7 years
Goodwill and intangible assets with indefinite lives				$ 313,481,000	313,481,000
Impairment recorded				0	0	$ 0
Carrying amount of goodwill arising from acquisitions, is deductible for federal income tax purposes				$ 13,640,000	$ 16,048,000
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				$ 99,253,000	$ 170,395,000
Minimum pension liability, included in accumulated other comprehensive income				0	$ 0
First financial antidilutive securities excluded from eps computation | shares					399,300
Subsequent Event [Member]
Equity Class Of Treasury Stock [Line Items]
Treasury Stock, Shares, Acquired | shares		0
Commitments to Extend Credit [Member]
Equity Class Of Treasury Stock [Line Items]
Reserve for unfunded commitments				6,436,000	$ 5,486,000
Held-to-maturity Securities [Member]
Equity Class Of Treasury Stock [Line Items]
Securities held to maturity				$ 0	0
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
Commercial Bancshares Inc and Commercial State Bank [Member]
Equity Class Of Treasury Stock [Line Items]
Percentage of common stock acquired										100.00%
Common Stock [Member]
Equity Class Of Treasury Stock [Line Items]
Shares repurchased under stock repurchase authorization, shares | shares			324,802
Treasury Stock, Value, Acquired			$ 8,008,000
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	2,000,000