INVESTMENT SECURITIES - Schedule of Investment Securities' Gross Unrealized Losses and Fair Value by Investment Category (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	$ 684,313	$ 398,776
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	17,848	3,624
Available-for-sale debt securities, 12 months or longer, Fair Value	231,577	239,678
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	14,141	8,024
Available-for-sale debt securities, Total, Fair Value	915,890	638,454
Available-for-sale debt securities, Total, Unrealized Loss	31,989	11,648
Held-to-maturity debt securities:
Held-to-maturity, Less than 12 months, Fair Value	103,069	23,404
Held-to-maturity, Less than 12 months, Unrealized Loss	897	59
Held-to-maturity, 12 months or longer, Fair Value	5,723	5,625
Held-to-maturity, 12 months or longer, Unrealized Loss	146	32
Held-to-maturity, Total, Fair Value	108,792	29,029
Held-to-maturity, Total, Unrealized Loss	1,043	91
CMO
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	199,985	146,726
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	5,031	1,820
Available-for-sale debt securities, 12 months or longer, Fair Value	83,359	77,149
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	4,267	2,321
Available-for-sale debt securities, Total, Fair Value	283,344	223,875
Available-for-sale debt securities, Total, Unrealized Loss	9,298	4,141
FNMA MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	397,186	204,921
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	10,438	1,479
Available-for-sale debt securities, 12 months or longer, Fair Value	114,832	126,342
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	7,898	4,693
Available-for-sale debt securities, Total, Fair Value	512,018	331,263
Available-for-sale debt securities, Total, Unrealized Loss	18,336	6,172
FHLMC MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	74,017	42,514
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	2,030	269
Available-for-sale debt securities, 12 months or longer, Fair Value	19,649	21,405
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	1,267	655
Available-for-sale debt securities, Total, Fair Value	93,666	63,919
Available-for-sale debt securities, Total, Unrealized Loss	3,297	924
GNMA MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	13,125	4,615
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	349	56
Available-for-sale debt securities, 12 months or longer, Fair Value	13,737	14,782
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	709	355
Available-for-sale debt securities, Total, Fair Value	26,862	19,397
Available-for-sale debt securities, Total, Unrealized Loss	1,058	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	103,069	23,404
Held-to-maturity, Less than 12 months, Unrealized Loss	897	59
Held-to-maturity, 12 months or longer, Fair Value	5,723	5,625
Held-to-maturity, 12 months or longer, Unrealized Loss	146	32
Held-to-maturity, Total, Fair Value	108,792	29,029
Held-to-maturity, Total, Unrealized Loss	$ 1,043	$ 91