INVESTMENT SECURITIES - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available for Sale Amortized Cost
Within one year	$ 0	$ 0
After one year but within five years	19,634	19,714
After five years but within ten years	165,299	170,118
After ten years	1,334,710	1,034,395
Amortized Cost	1,519,643	1,224,227
Available for Sale Fair Value
Within one year	0	0
After one year but within five years	19,545	19,423
After five years but within ten years	162,596	163,731
After ten years	1,341,055	1,021,925
Available-for-sale securities, fair value total	1,523,196	1,205,079
Held to Maturity, Amortized Cost
Within one year	1,135	1,018
After one year but within five years	8,950	6,703
After five years but within ten years	29,992	29,613
After ten years	108,113	112,616
Amortized Cost	148,190	149,950
Held to Maturity, Fair Value
Within one year	1,135	1,016
After one year but within five years	8,971	6,701
After five years but within ten years	30,240	29,547
After ten years	109,386	112,167
Held-to-maturity securities, fair value total	$ 149,732	$ 149,431