INVESTMENTS (Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
AFS Investments (carried at fair value):
Available-for-sale securities, at fair value	$ 780,343	$ 789,899
HTM Investments (carried at amortized cost):
Amortized Cost	92,933	94,073
Unrealized Gains	58	1,077
Unrealized Losses	(2,441)	(237)
Fair Value	90,550	94,913
Obligations of states and political subdivisions
AFS Investments (carried at fair value):
Amortized Cost	4,778	7,232
Unrealized Gains	44	103
Unrealized Losses	(4)	0
Available-for-sale securities, at fair value	4,818	7,335
HTM Investments (carried at amortized cost):
Amortized Cost	92,933	94,073
Unrealized Gains	58	1,077
Unrealized Losses	(2,441)	(237)
Fair Value	90,550	94,913
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
AFS Investments (carried at fair value):
Amortized Cost	479,836	510,176
Unrealized Gains	350	597
Unrealized Losses	(19,888)	(7,471)
Available-for-sale securities, at fair value	460,298	503,302
Collateralized mortgage obligations issued or guaranteed by U.S. government-sponsored enterprises
AFS Investments (carried at fair value):
Amortized Cost	320,543	279,575
Unrealized Gains	0	14
Unrealized Losses	(13,932)	(6,790)
Available-for-sale securities, at fair value	306,611	272,799
Subordinated corporate bonds
AFS Investments (carried at fair value):
Amortized Cost	8,589	5,484
Unrealized Gains	65	173
Unrealized Losses	(38)	0
Available-for-sale securities, at fair value	8,616	5,657
Equity investments
AFS Investments (carried at fair value):
Amortized Cost		554
Unrealized Gains		252
Unrealized Losses		0
Available-for-sale securities, at fair value		806
Total AFS investments
AFS Investments (carried at fair value):
Amortized Cost	813,746	803,021
Unrealized Gains	459	1,139
Unrealized Losses	(33,862)	(14,261)
Available-for-sale securities, at fair value	$ 780,343	$ 789,899