INVESTMENTS (Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Abstract]
Fair Value	$ 1,115,548	$ 1,115,813
Obligations of U.S. government sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	5,200
Unrealized Gains	0
Unrealized Losses	(217)
Fair Value	4,983
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	114,280	119,608
Unrealized Gains	5,982	7,627
Unrealized Losses	(463)	(115)
Fair Value	119,799	127,120
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	564,151	547,396
Unrealized Gains	10,581	19,796
Unrealized Losses	(5,413)	(574)
Fair Value	569,319	566,618
Collateralized mortgage obligations issued or guaranteed by U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	406,358	399,937
Unrealized Gains	6,452	10,652
Unrealized Losses	(2,166)	(135)
Fair Value	410,644	410,454
Subordinated corporate bonds
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	10,525	11,533
Unrealized Gains	293	186
Unrealized Losses	(15)	(98)
Fair Value	10,803	11,621
Total AFS debt securities
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	1,100,514	1,078,474
Unrealized Gains	23,308	38,261
Unrealized Losses	(8,274)	(922)
Fair Value	$ 1,115,548	$ 1,115,813