Fair Market Value of Financial Assets and Liabilities, Accompanying Balance Sheets Measured at Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	$ 1,525,738			$ 1,525,738		$ 997,261
Equity securities at fair value	2,461			2,461		2,471
Assets measured-nonrecurring basis [Abstract]
Collateral dependent loans	1,264			1,264		1,768
Reconciliation of beginning and ending balances of recurring fair value measurements recognized in balance sheet using significant unobservable (Level 3) inputs [Roll Forward]
Total gains (losses)	(103)		$ 99	$ 370	$ (1,066)
Minimum [Member]
Other real estate owned [Abstract]
Typical frequency of periodic reviews				12 months
Maximum [Member]
Other real estate owned [Abstract]
Typical frequency of periodic reviews				18 months
Frequency of periodic reviews in general				24 months
U.S. Treasury and Government Agencies [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	302,410			$ 302,410		148,793
State and Political Subdivisions [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	335,556			335,556		140,416
U.S. Government Sponsored Agency Mortgage-backed Securities [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	787,507			787,507		651,807
Asset-backed Securities [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	100,265			100,265		56,245
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	244,361			244,361		74,991
Equity securities at fair value	0			0		0
Significant Other Observable Inputs (Level 2) [Member] | Fair Value [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	1,281,377			1,281,377		922,270
Equity securities at fair value	0			0		0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	0			0		0
Equity securities at fair value	2,461			2,461		2,471
Recurring [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Equity securities at fair value	2,461			2,461		2,471
Mortgage servicing rights	6,265			6,265		4,068
Recurring [Member] | U.S. Treasury and Government Agencies [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	302,410			302,410		148,793
Recurring [Member] | State and Political Subdivisions [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	335,556			335,556		140,416
Recurring [Member] | U.S. Government Sponsored Agency Mortgage-backed Securities [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	787,507			787,507		651,807
Recurring [Member] | Asset-backed Securities [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	100,265			100,265		56,245
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Equity securities at fair value	0			0		0
Mortgage servicing rights	0			0		0
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury and Government Agencies [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	244,361			244,361		74,991
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | State and Political Subdivisions [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	0			0		0
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Government Sponsored Agency Mortgage-backed Securities [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	0			0		0
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Asset-backed Securities [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	0			0		0
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Equity securities at fair value	0			0		0
Mortgage servicing rights	0			0		0
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury and Government Agencies [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	58,049			58,049		73,802
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | State and Political Subdivisions [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	335,556			335,556		140,416
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Sponsored Agency Mortgage-backed Securities [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	787,507			787,507		651,807
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Asset-backed Securities [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	100,265			100,265		56,245
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Equity securities at fair value	2,461			2,461		2,471,000
Mortgage servicing rights	6,265			6,265		4,068
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury and Government Agencies [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	0			0		0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | State and Political Subdivisions [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	0			0		0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Government Sponsored Agency Mortgage-backed Securities [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	0			0		0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities [Member]
Assets Measured at Fair Value on Recurring Basis [Abstract]
Debt securities available-for-sale	0			0		0
Recurring [Member] | Equity Securities [Member]
Reconciliation of beginning and ending balances of recurring fair value measurements recognized in balance sheet using significant unobservable (Level 3) inputs [Roll Forward]
Beginning balance	2,523		2,094	2,471	1,953	1,953
Total unrealized gains (losses) included in net income	(62)		118	(10)	259
Issues	0		0	0	0
Settlements	0		0	0	0
Ending balance	2,461	$ 2,523	2,212	2,461	2,212	2,471
Total gains (losses) for the period included in net income attributable to the change in unrealized gains or losses related to assets still held at the reporting date	(62)		119	(10)	259
Recurring [Member] | Mortgage Servicing Rights [Member]
Reconciliation of beginning and ending balances of recurring fair value measurements recognized in balance sheet using significant unobservable (Level 3) inputs [Roll Forward]
Beginning balance	5,899		2,518	4,068	3,263	3,263
Total unrealized gains (losses) included in net income	141		286	1,042	(680)
Issues	407		611	1,817	1,171
Settlements	(182)		(306)	(662)	(645)
Ending balance	6,265	5,899	3,109	6,265	3,109	4,068
Total gains (losses) for the period included in net income attributable to the change in unrealized gains or losses related to assets still held at the reporting date	141		286	1,042	(680)
Nonrecurring [Member]
Impaired loan (collateral dependent) [Abstract]
Fair value adjustments on collateral dependent loans	100	500	400	800	1,700	500
Other real estate owned [Abstract]
Other real estate owned, fair value adjustment	100	$ 300	$ 200	100	$ 600	700
Nonrecurring [Member] | Fair Value [Member]
Assets measured-nonrecurring basis [Abstract]
Collateral dependent loans	1,264			1,264		1,768
Other real estate owned	522			522		2,395
Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value [Member]
Assets measured-nonrecurring basis [Abstract]
Collateral dependent loans	0			0		0
Other real estate owned	0			0		0
Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value [Member]
Assets measured-nonrecurring basis [Abstract]
Collateral dependent loans	0			0		0
Other real estate owned	0			0		0
Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value [Member]
Assets measured-nonrecurring basis [Abstract]
Collateral dependent loans	1,264			1,264		1,768
Other real estate owned	$ 522			$ 522		$ 2,395