Fair Value of Assets and Liabilities - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale securities:
Total available-for-sale securities	$ 737,227,000	$ 685,636,000
Equity securities, at fair value	185,000	412,000
U.S. Treasury Securities and Obligations of U.S. Government Corporations & Agencies
Available-for-sale securities:
Total available-for-sale securities	94,866,000	107,320,000
Obligations of States and Political Subdivisions
Available-for-sale securities:
Total available-for-sale securities	232,935,000	178,433,000
Mortgage-backed Securities
Available-for-sale securities:
Total available-for-sale securities	399,300,000	396,126,000
Other Securities
Available-for-sale securities:
Total available-for-sale securities	10,126,000	3,757,000
Fair Value, Recurring
Available-for-sale securities:
Total available-for-sale securities	737,227,000	685,636,000
Equity securities, at fair value	185,000	412,000
Derivative assets, Interest rate swaps	1,825,000
Total assets	739,237,000	686,048,000
Derivative liabilities, Interest rate swaps	3,574,000	325,000
Fair Value, Recurring | U.S. Treasury Securities and Obligations of U.S. Government Corporations & Agencies
Available-for-sale securities:
Total available-for-sale securities	94,866,000	107,320,000
Fair Value, Recurring | Obligations of States and Political Subdivisions
Available-for-sale securities:
Total available-for-sale securities	232,935,000	178,433,000
Fair Value, Recurring | Mortgage-backed Securities
Available-for-sale securities:
Total available-for-sale securities	399,300,000	396,126,000
Fair Value, Recurring | Other Securities
Available-for-sale securities:
Total available-for-sale securities	10,126,000	3,757,000
Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale securities:
Total available-for-sale securities	0	0
Equity securities, at fair value	185,000	219,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Available-for-sale securities:
Total available-for-sale securities	0	0
Equity securities, at fair value	185,000	219,000
Derivative assets, Interest rate swaps	0
Total assets	185,000	219,000
Derivative liabilities, Interest rate swaps	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring | U.S. Treasury Securities and Obligations of U.S. Government Corporations & Agencies
Available-for-sale securities:
Total available-for-sale securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring | Obligations of States and Political Subdivisions
Available-for-sale securities:
Total available-for-sale securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring | Mortgage-backed Securities
Available-for-sale securities:
Total available-for-sale securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring | Other Securities
Available-for-sale securities:
Total available-for-sale securities	0	0
Significant Other Observable Inputs (Level 2)
Available-for-sale securities:
Total available-for-sale securities	736,434,000	684,856,000
Equity securities, at fair value	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Available-for-sale securities:
Total available-for-sale securities	736,434,000	684,663,000
Equity securities, at fair value	0	193,000
Derivative assets, Interest rate swaps	1,825,000
Total assets	738,259,000	684,856,000
Derivative liabilities, Interest rate swaps	3,574,000	325,000
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | U.S. Treasury Securities and Obligations of U.S. Government Corporations & Agencies
Available-for-sale securities:
Total available-for-sale securities	94,866,000	107,320,000
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | Obligations of States and Political Subdivisions
Available-for-sale securities:
Total available-for-sale securities	232,142,000	177,460,000
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | Mortgage-backed Securities
Available-for-sale securities:
Total available-for-sale securities	399,300,000	396,126,000
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | Other Securities
Available-for-sale securities:
Total available-for-sale securities	10,126,000	3,757,000
Significant Unobservable Inputs (Level 3)
Available-for-sale securities:
Total available-for-sale securities	793,000	780,000
Equity securities, at fair value	0	193,000
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring
Available-for-sale securities:
Total available-for-sale securities	793,000	973,000
Equity securities, at fair value	0	0
Derivative assets, Interest rate swaps	0
Total assets	793,000	973,000
Derivative liabilities, Interest rate swaps	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring | U.S. Treasury Securities and Obligations of U.S. Government Corporations & Agencies
Available-for-sale securities:
Total available-for-sale securities	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring | Obligations of States and Political Subdivisions
Available-for-sale securities:
Total available-for-sale securities	793,000	973,000
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring | Mortgage-backed Securities
Available-for-sale securities:
Total available-for-sale securities	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring | Other Securities
Available-for-sale securities:
Total available-for-sale securities	$ 0	$ 0