Summary of Significant Accounting Policies - Additional Information (Detail)		1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 23, 2019 shares	Apr. 21, 2020 USD ($) shares	Mar. 31, 2020 USD ($) Location shares	Mar. 31, 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
Minimum number of shares that company is required to repurchase | shares			0
Stock repurchased under authorization | shares			0
Minimum number of days to consider the loans as non-accrual			90 days
Loans individually evaluated for impairment			$ 39,226,000	$ 28,508,000	$ 24,582,000
Contractual balance			53,284,000	38,336,000	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			$ 123,576,000	31,830,000
Minimum pension liability, included in accumulated other comprehensive income			0	(1,324,000)
Stock compensation expense			$ 341,000	312,000
Treasury Stock, Shares, Acquired | shares			0
Subsequent Event [Member]
Equity, Class of Treasury Stock [Line Items]
Stock repurchased under authorization | shares		263,464
Treasury Stock, Shares, Acquired | shares		263,464
Treasury Stock, Value, Acquired		$ 6,480,000,000
Purchased Credit Impaired Loans [Member]
Equity, Class of Treasury Stock [Line Items]
Loans individually evaluated for impairment			$ 7,773,000	859,000	251,000
Contractual balance			10,411,000	1,196,000
Officers [Member]
Equity, Class of Treasury Stock [Line Items]
Restricted stock expense			$ 450,000	$ 340,000	$ 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