Recent Accounting Pronouncements (Impact of ASC 326 in Loan Portfolio Segments as January 1, 2020) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	$ 3,012,989	$ 3,091,998		$ 3,081,973
Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans				0
December 31, 2019 After Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans				3,081,973
Commercial and Industrial Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	510,583	638,773	$ 534,692	589,758
Commercial and Industrial Loans | Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans			2,191	(57,257)
Commercial and Industrial Loans | December 31, 2019 After Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans				532,501
Commercial Real Estate Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	1,528,493	1,467,397	1,500,247	1,495,862
Commercial Real Estate Loans | Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans			4,385
Commercial Real Estate Loans | December 31, 2019 After Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans				1,495,862
Agricultural Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	349,321	376,186	384,654	384,526
Agricultural Loans | Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans			128
Agricultural Loans | December 31, 2019 After Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans				384,526
Leases
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	56,186	55,664	57,257
Leases | Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans			0	57,257
Leases | December 31, 2019 After Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans				57,257
Home Equity Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	217,994	219,348	225,790	225,755
Home Equity Loans | Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans			35
Home Equity Loans | December 31, 2019 After Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans				225,755
Consumer Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	67,150	66,717	69,264	81,217
Consumer Loans | Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans			0	(11,953)
Consumer Loans | December 31, 2019 After Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans				69,264
Credit Cards
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	13,856	11,637	11,953
Credit Cards | Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans			0	11,953
Credit Cards | December 31, 2019 After Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans				11,953
Residential Mortgage Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	$ 269,406	$ 256,276	305,002	304,855
Residential Mortgage Loans | Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans			$ 147
Residential Mortgage Loans | December 31, 2019 After Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans				$ 304,855