Investment Securities - Available for Sale and Held for Maturity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale: [Abstract]
Amortized Cost	$ 713,288	$ 673,945
Gross Unrealized Gains	24,422	12,280
Gross Unrealized (Losses)	(483)	(589)
Fair Value	737,227	685,636
Held-to-maturity: [Abstract]
Held-to-maturity	10,015	69,542
Fair Value	10,162	69,572
U.S. Treasury Securities and Obligations of U.S. Government Corporations & Agencies
Available-for-sale: [Abstract]
Amortized Cost	93,990	106,428
Gross Unrealized Gains	1,011	952
Gross Unrealized (Losses)	(135)	(60)
Fair Value	94,866	107,320
Held-to-maturity: [Abstract]
Held-to-maturity	10,015	69,542
Gross Unrealized Gains	147	99
Unrealized (Losses)	0	(69)
Fair Value	10,162	69,572
Obligations of States and Political Subdivisions
Available-for-sale: [Abstract]
Amortized Cost	223,082	172,460
Gross Unrealized Gains	10,188	5,990
Gross Unrealized (Losses)	(335)	(17)
Fair Value	232,935	178,433
Mortgage-backed Securities: GSE Residential
Available-for-sale: [Abstract]
Amortized Cost	386,226	391,307
Gross Unrealized Gains	13,082	5,331
Gross Unrealized (Losses)	(8)	(512)
Fair Value	399,300	396,126
Other Securities
Available-for-sale: [Abstract]
Amortized Cost	9,990	3,750
Gross Unrealized Gains	141	7
Gross Unrealized (Losses)	(5)	0
Fair Value	$ 10,126	$ 3,757

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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