Investment Securities - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Available-for-sale investments, One year or less	$ 165,138
Available-for-sale investments, After 1 through 5 years	366,233
Available-for-sale investments, After 5 through 10 years	203,930
Available-for-sale investments, After ten years	1,926
Available-for-sale investments, Fair Value	$ 737,227	$ 685,636
Available-for-sale, Weighted average yield, One year or less (in hundredths)	2.02%
Available-for-sale , Weighted average yield,After 1 through 5 years (in hundredths)	2.61%
Available-for-sale, Weighted average yield,After 5 through 10 years (in hundredths)	2.18%
Available-for-sale , Weighted average yield,After ten years (in hundredths)	3.58%
Available-for-sale , Weighted average yield,Total (in hundredths)	2.36%
Available-for-sale, Full Tax-equivalent yield, One year or less (in hundredths)	2.22%
Available-for-sale, Full Tax-equivalent yield,After 1 through 5 years (in hundredths)	2.83%
Available-for-sale, Full Tax-equivalent yield, After 5 through 10 years (in hundredths)	2.72%
Available-for-sale, Full Tax-equivalent yield,After ten years (in hundredths)	4.89%
Available-for-sale, Full Tax-equivalent yield,Total (in hundredths)	2.67%
Held-to-maturity, Maturity [Abstract]
Held-to-maturity	$ 10,015	69,542
Held To Maturity Weighted average yield, One year or less	2.53%
Held To Maturity Weighted average yield, After 1 through 5 years	2.06%
Held To Maturity Weighted average yield After 5 through 10 years	0.00%
Held To Maturity Weighted average yield After ten years	0.00%
Held To Maturity Weighted average yield ,Total	2.29%
Held To Maturity, Full tax-equivalent yield , One year or less	2.53%
Held To Maturity, Full tax-equivalent yield , After 1 through 5 years	2.06%
Held To Maturity, Full tax-equivalent yield , After 5 through 10 years	0.00%
Held To Maturity, Full tax-equivalent yield , After ten years	0.00%
Held To Maturity, Full tax-equivalent yield , Total	2.29%
U.S. Treasury Securities and Obligations of U.S. Government Corporations & Agencies
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Available-for-sale investments, One year or less	$ 67,365
Available-for-sale investments, After 1 through 5 years	15,561
Available-for-sale investments, After 5 through 10 years	11,940
Available-for-sale investments, After ten years	0
Available-for-sale investments, Fair Value	94,866	107,320
Held-to-maturity, Maturity [Abstract]
Held-to-maturity, One Year or less	4,995
Held-to-maturity, After1 through 5 years	5,020
Held-to-maturity, After 5 through 10 years	0
Held-to-maturity, After ten years	0
Held-to-maturity	10,015	69,542
Obligations of States and Political Subdivisions
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Available-for-sale investments, One year or less	30,007
Available-for-sale investments, After 1 through 5 years	75,417
Available-for-sale investments, After 5 through 10 years	125,585
Available-for-sale investments, After ten years	1,926
Available-for-sale investments, Fair Value	232,935	178,433
Mortgage-backed Securities: GSE Residential
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Available-for-sale investments, One year or less	67,766
Available-for-sale investments, After 1 through 5 years	266,128
Available-for-sale investments, After 5 through 10 years	65,406
Available-for-sale investments, After ten years	0
Available-for-sale investments, Fair Value	399,300	396,126
Other Securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Available-for-sale investments, One year or less	0
Available-for-sale investments, After 1 through 5 years	9,127
Available-for-sale investments, After 5 through 10 years	999
Available-for-sale investments, After ten years	0
Available-for-sale investments, Fair Value	$ 10,126	$ 3,757