Investment Securities - Available for Sale and Held for Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale: [Abstract]
Amortized Cost	$ 855,163	$ 673,946
Gross Unrealized Gains	25,075	12,279
Gross Unrealized (Losses)	(998)	(589)
Fair Value	879,240	685,636
Held-to-maturity: [Abstract]
Held-to-maturity	5,016	69,542
Fair Value	5,119	69,572
U.S. Treasury Securities and Obligations of U.S. Government Corporations & Agencies
Available-for-sale: [Abstract]
Amortized Cost	127,067	106,428
Gross Unrealized Gains	790	952
Gross Unrealized (Losses)	(788)	(60)
Fair Value	127,069	107,320
Held-to-maturity: [Abstract]
Held-to-maturity	5,016	69,542
Gross Unrealized Gains	103	99
Unrealized (Losses)	0	(69)
Fair Value	5,119	69,572
Obligations of States and Political Subdivisions
Available-for-sale: [Abstract]
Amortized Cost	237,886	172,460
Gross Unrealized Gains	11,995	5,990
Gross Unrealized (Losses)	(37)	(17)
Fair Value	249,844	178,433
Mortgage-backed Securities: GSE Residential
Available-for-sale: [Abstract]
Amortized Cost	479,470	391,307
Gross Unrealized Gains	12,038	5,331
Gross Unrealized (Losses)	(160)	(512)
Fair Value	491,348	396,126
Other Securities
Available-for-sale: [Abstract]
Amortized Cost	10,740	3,751
Gross Unrealized Gains	252	6
Gross Unrealized (Losses)	(13)	0
Fair Value	$ 10,979	$ 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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