INVESTMENTS (Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
AFS Investments (carried at fair value):
Fair Value	$ 1,107,069	$ 918,118
HTM Investments (carried at amortized cost):
Amortized Cost	1,298	1,302
Unrealized Gains	105	57
Unrealized Losses	0	0
Fair Value	1,403	1,359
Obligations of states and political subdivisions
AFS Investments (carried at fair value):
Amortized Cost	120,636	115,632
Unrealized Gains	7,055	2,779
Unrealized Losses	(78)	(328)
Fair Value	127,613	118,083
HTM Investments (carried at amortized cost):
Amortized Cost	1,298	1,302
Unrealized Gains	105	57
Unrealized Losses	0	0
Fair Value	1,403	1,359
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
AFS Investments (carried at fair value):
Amortized Cost	549,930	462,593
Unrealized Gains	19,036	3,398
Unrealized Losses	(864)	(2,605)
Fair Value	568,102	463,386
Collateralized mortgage obligations issued or guaranteed by U.S. government-sponsored enterprises
AFS Investments (carried at fair value):
Amortized Cost	389,382	325,200
Unrealized Gains	11,313	3,183
Unrealized Losses	(129)	(2,478)
Fair Value	400,566	325,905
Subordinated corporate bonds
AFS Investments (carried at fair value):
Amortized Cost	10,531	10,553
Unrealized Gains	313	191
Unrealized Losses	(56)	0
Fair Value	10,788	10,744
Total AFS investments
AFS Investments (carried at fair value):
Amortized Cost	1,070,479	913,978
Unrealized Gains	37,717	9,551
Unrealized Losses	(1,127)	(5,411)
Fair Value	$ 1,107,069	$ 918,118