Basis of Accounting and Consolidation - Additional Information (Details) - USD ($)				3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 06, 2020	Sep. 25, 2020	Jan. 02, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jun. 30, 2020	Apr. 21, 2020	Jan. 01, 2020	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Basis Of Accounting And Consolidation [Line Items]
Common stock, par value (in dollars per share)				$ 4		$ 4		$ 4
Loans acquired										$ 183,000,000
Stock plans, term						10 years
Maximum number of shares to be issued in stock incentive plan (in shares)				149,983		149,983
Employee discount for employee stock purchase plan						5.00%
Employee stock purchase plan, number of shares authorized				600,000		600,000
Stock issued during period, shares, employee stock purchase plans				3,494	1,976	11,037	4,834
Captive maximum gross premiums						$ 2,300,000
Right of use lease assets				$ 16,703,000	$ 12,544,000	16,703,000	$ 12,544,000	$ 17,006,000
Lease liabilities				16,817,000	12,557,000	16,817,000	12,557,000	17,007,000
Cumulative change in accounting principal				561,009,000	521,959,000	561,009,000	521,959,000	526,609,000	$ 549,273,000			$ 508,958,000		$ 475,864,000
Allowance for credit losses on loans				$ 41,915,000	$ 26,741,000	$ 41,915,000	$ 26,741,000	26,911,000	$ 38,381,000			$ 26,359,000		$ 26,189,000
ASU 2016-02
Basis Of Accounting And Consolidation [Line Items]
Right of use lease assets													$ 14,100,000
Lease liabilities													$ 14,100,000
ASU 2016-13
Basis Of Accounting And Consolidation [Line Items]
Allowance for credit losses on loans											$ 28,583,000
Off-balance sheet, credit loss, liability											69,000
Allowance for credit losses, held-to-maturity debt securities											0
Amortized cost of PCD assets reflected the addition to allowance for credit losses			$ 833,000
ASU 2016-13 | Impact of ASU
Basis Of Accounting And Consolidation [Line Items]
Allowance for credit losses on loans											1,672,000
Off-balance sheet, credit loss, liability											69,000
ASU 2016-13 | Cumulative Change in Accounting Principal, Adjustment
Basis Of Accounting And Consolidation [Line Items]
Cumulative change in accounting principal								(717,000)			$ 717,000
Allowance for credit losses on loans								$ 1,672,000
Stock Options
Basis Of Accounting And Consolidation [Line Items]
Common stock awarded in SI plan (in shares)						0
Restricted Stock
Basis Of Accounting And Consolidation [Line Items]
Common stock awarded in SI plan (in shares)						25,200		25,950
Restricted Stock Units (RSUs)
Basis Of Accounting And Consolidation [Line Items]
Common stock awarded in SI plan (in shares)						16,950		15,540
3.95% Subordinated Notes due 2030 | Subsequent Event
Basis Of Accounting And Consolidation [Line Items]
Aggregate principal amount	$ 96,000,000.0
Fixed-to-Floating Rate	3.95%
Maturity date	Oct. 15, 2030
Redemption description	The Company may, beginning with the interest payment date of October 15, 2025, and on any interest payment date thereafter, redeem the Notes, in whole or in part, at a redemption price equal to 100% of the principal amount of the Notes to be redeemed plus accrued and unpaid interest to but excluding the date of redemption. The Company may also redeem the Notes at any time, including prior to October 15, 2025, at the Company’s option, in whole but not in part, if: (i) a change or prospective change in law occurs that could prevent the Company from deducting interest payable on the Notes for U.S. federal income tax purposes; (ii) a subsequent event occurs that could preclude the Notes from being recognized as Tier 2 capital for regulatory capital purposes; or (iii) the Company is required to register as an investment company under the Investment Company Act of 1940, as amended; in each case, at a redemption price equal to 100% of the principal amount of the Notes plus any accrued and unpaid interest to but excluding the redemption date.
3.95% Subordinated Notes due 2030 | SOFR | Subsequent Event
Basis Of Accounting And Consolidation [Line Items]
Description floating rate basis	three-month Term SOFR
Basis points	3.83%
LINCO Bancshares, Inc.
Basis Of Accounting And Consolidation [Line Items]
Common stock, par value (in dollars per share)		$ 1.00
Share Price		$ 4.00
Consideration payable in cash		$ 116,500,000
Consideration payable in shares		1,262,246
Dividends		$ 13,000,000