Fair Value of Assets and Liabilities - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Available-for-sale securities:
Total available-for-sale securities	$ 1,222,198,000	$ 879,240,000
Equity securities, at fair value	301,000	218,000
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies
Available-for-sale securities:
Total available-for-sale securities	192,179,000	127,069,000
Obligations of States and Political Subdivisions
Available-for-sale securities:
Total available-for-sale securities	305,626,000	249,844,000
Mortgage-backed Securities
Available-for-sale securities:
Total available-for-sale securities	688,570,000	491,348,000
Other Securities
Available-for-sale securities:
Total available-for-sale securities	35,823,000	10,979,000
Fair Value, Recurring
Available-for-sale securities:
Total available-for-sale securities	1,222,198,000	879,240,000
Equity securities, at fair value	301,000	218,000
Derivative assets, Interest rate swaps	181,000	1,399,000
Total assets	1,222,680,000	880,857,000
Derivative liabilities, Interest rate swaps	1,083,000	2,892,000
Fair Value, Recurring | U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies
Available-for-sale securities:
Total available-for-sale securities	192,179,000	127,069,000
Fair Value, Recurring | Obligations of States and Political Subdivisions
Available-for-sale securities:
Total available-for-sale securities	305,626,000	249,844,000
Fair Value, Recurring | Mortgage-backed Securities
Available-for-sale securities:
Total available-for-sale securities	688,570,000	491,348,000
Fair Value, Recurring | Other Securities
Available-for-sale securities:
Total available-for-sale securities	35,823,000	10,979,000
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	301,000	218,000
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	301,000	218,000
Derivative assets, Interest rate swaps	0	0
Total assets	301,000	218,000
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 and 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	1,221,583,000	878,446,000
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	1,221,583,000	878,446,000
Equity securities, at fair value	0	193,000
Derivative assets, Interest rate swaps	181,000	1,399,000
Total assets	1,221,764,000	880,038,000
Derivative liabilities, Interest rate swaps	1,083,000	2,892,000
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies
Available-for-sale securities:
Total available-for-sale securities	192,179,000	127,069,000
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | Obligations of States and Political Subdivisions
Available-for-sale securities:
Total available-for-sale securities	305,011,000	249,050,000
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | Mortgage-backed Securities
Available-for-sale securities:
Total available-for-sale securities	688,570,000	491,348,000
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | Other Securities
Available-for-sale securities:
Total available-for-sale securities	35,823,000	10,979,000
Significant Unobservable Inputs (Level 3)
Available-for-sale securities:
Total available-for-sale securities	615,000	794,000
Equity securities, at fair value	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring
Available-for-sale securities:
Total available-for-sale securities	615,000	794,000
Equity securities, at fair value	0	0
Derivative assets, Interest rate swaps	0	0
Total assets	615,000	794,000
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 and 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	615,000	794,000
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