INVESTMENT SECURITIES - Schedule of Investment Securities' Gross Unrealized Losses and Fair Value by Investment Category (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	$ 517,055	$ 398,776
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	12,648	3,624
Available-for-sale debt securities, 12 months or longer, Fair Value	472,361	239,678
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	27,200	8,024
Available-for-sale debt securities, Total, Fair Value	989,416	638,454
Available-for-sale debt securities, Total, Unrealized Loss	39,848	11,648
Held-to-maturity debt securities:
Held-to-maturity, Less than 12 months, Fair Value	150,060	23,404
Held-to-maturity, Less than 12 months, Unrealized Loss	2,013	59
Held-to-maturity, 12 months or longer, Fair Value	523	5,625
Held-to-maturity, 12 months or longer, Unrealized Loss	36	32
Held-to-maturity, Total, Fair Value	150,583	29,029
Held-to-maturity, Total, Unrealized Loss	2,049	91
CMO
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	138,346	146,726
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	2,826	1,820
Available-for-sale debt securities, 12 months or longer, Fair Value	161,684	77,149
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	8,651	2,321
Available-for-sale debt securities, Total, Fair Value	300,030	223,875
Available-for-sale debt securities, Total, Unrealized Loss	11,477	4,141
FNMA MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	303,994	204,921
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	8,047	1,479
Available-for-sale debt securities, 12 months or longer, Fair Value	244,744	126,342
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	14,901	4,693
Available-for-sale debt securities, Total, Fair Value	548,738	331,263
Available-for-sale debt securities, Total, Unrealized Loss	22,948	6,172
FHLMC MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	55,386	42,514
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	1,271	269
Available-for-sale debt securities, 12 months or longer, Fair Value	52,514	21,405
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	2,841	655
Available-for-sale debt securities, Total, Fair Value	107,900	63,919
Available-for-sale debt securities, Total, Unrealized Loss	4,112	924
GNMA MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	19,329	4,615
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	504	56
Available-for-sale debt securities, 12 months or longer, Fair Value	13,419	14,782
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	807	355
Available-for-sale debt securities, Total, Fair Value	32,748	19,397
Available-for-sale debt securities, Total, Unrealized Loss	1,311	411
Other equity investments
Other equity investments
Other investments, Less than 12 months, Fair Value		0
Other investments, Less than 12 months, Unrealized Loss		0
Other investments, 12 months or longer, Fair Value		624
Other investments, 12 months or longer, Unrealized Loss		20
Other investments, Total, Fair Value		624
Other investments, Total, Unrealized Loss		20
State and political subdivisions
Held-to-maturity debt securities:
Held-to-maturity, Less than 12 months, Fair Value	150,060	23,404
Held-to-maturity, Less than 12 months, Unrealized Loss	2,013	59
Held-to-maturity, 12 months or longer, Fair Value	523	5,625
Held-to-maturity, 12 months or longer, Unrealized Loss	36	32
Held-to-maturity, Total, Fair Value	150,583	29,029
Held-to-maturity, Total, Unrealized Loss	$ 2,049	$ 91