INVESTMENT SECURITIES - Schedule of Investment Securities' Gross Unrealized Losses and Fair Value by Investment Category (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	$ 6,875	$ 73,219
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	26	302
Available-for-sale debt securities, 12 months or longer, Fair Value	607,612	718,120
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	9,348	22,748
Available-for-sale debt securities, Total, Fair Value	614,487	791,339
Available-for-sale debt securities, Total, Unrealized Loss	9,374	23,050
Held-to-maturity debt securities:
Held-to-maturity, Less than 12 months, Fair Value	135,232
Held-to-maturity, Less than 12 months, Unrealized Loss	9
Held-to-maturity, 12 months or longer, Fair Value	14,500
Held-to-maturity, 12 months or longer, Unrealized Loss	42
Held-to-maturity, Total, Fair Value	149,732
Held-to-maturity, Total, Unrealized Loss	51
CMO
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	3,896	17,143
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	21	40
Available-for-sale debt securities, 12 months or longer, Fair Value	196,623	212,208
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	3,424	6,798
Available-for-sale debt securities, Total, Fair Value	200,519	229,351
Available-for-sale debt securities, Total, Unrealized Loss	3,445	6,838
FNMA MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	0	34,214
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	0	162
Available-for-sale debt securities, 12 months or longer, Fair Value	330,219	407,638
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	4,734	12,776
Available-for-sale debt securities, Total, Fair Value	330,219	441,852
Available-for-sale debt securities, Total, Unrealized Loss	4,734	12,938
FHLMC MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	0	16,025
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	0	21
Available-for-sale debt securities, 12 months or longer, Fair Value	63,285	76,469
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	747	2,373
Available-for-sale debt securities, Total, Fair Value	63,285	92,494
Available-for-sale debt securities, Total, Unrealized Loss	747	2,394
GNMA MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	2,979	5,837
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	5	79
Available-for-sale debt securities, 12 months or longer, Fair Value	17,485	21,805
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	443	801
Available-for-sale debt securities, Total, Fair Value	20,464	27,642
Available-for-sale debt securities, Total, Unrealized Loss	448	880
State and political subdivisions
Held-to-maturity debt securities:
Held-to-maturity, Less than 12 months, Fair Value	134,732	91,228
Held-to-maturity, Less than 12 months, Unrealized Loss	9	155
Held-to-maturity, 12 months or longer, Fair Value	14,500	58,203
Held-to-maturity, 12 months or longer, Unrealized Loss	42	639
Held-to-maturity, Total, Fair Value	149,232	149,431
Held-to-maturity, Total, Unrealized Loss	51	$ 794
Foreign bonds
Held-to-maturity debt securities:
Held-to-maturity, Less than 12 months, Fair Value	500
Held-to-maturity, Less than 12 months, Unrealized Loss	0
Held-to-maturity, 12 months or longer, Fair Value	0
Held-to-maturity, 12 months or longer, Unrealized Loss	0
Held-to-maturity, Total, Fair Value	500
Held-to-maturity, Total, Unrealized Loss	$ 0

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months.

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