Summary of Significant Accounting Policies - Additional Information (Detail)		3 Months Ended		12 Months Ended
	May 05, 2021 USD ($)	Mar. 31, 2021 USD ($) Location shares	Mar. 31, 2020 USD ($) shares	Dec. 31, 2020 USD ($)	Sep. 30, 2020 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
Minimum number of days to consider the loans as non-accrual		90 days
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		$ 117,009,000	123,576,000	$ 170,395,000
Retained earnings net of tax after transitional charges		$ 875,147,000	$ 727,828,000	836,729,000		$ 466,000
First financial antidilutive securities excluded from eps computation | shares		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
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased under stock repurchase authorization, shares | shares		324,802
Cumulative Effect, Period of Adoption, Adjustment [Member]
Equity, Class of Treasury Stock [Line Items]
Retained earnings net of tax after transitional charges						$ 589,000
Subsequent Event [Member]
Equity, Class of Treasury Stock [Line Items]
Treasury Stock, Value, Acquired	$ 8,008,000
Commitments to Extend Credit [Member]
Equity, Class of Treasury Stock [Line Items]
Reserve for unfunded commitments		$ 6,918,000	$ 809,000	5,486,000
Held-to-maturity Securities [Member]
Equity, Class of Treasury Stock [Line Items]
Securities held to maturity		$ 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					2,000,000		4,000,000