INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Held-to-Maturity Amortized Cost
Amortized Cost	$ 136,744	$ 142,862
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	140,065	142,821
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	2,857,355	2,798,298
Available-for-Sale Market Value
Investment securities available-for-sale	2,908,688
One Year or Less [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	0
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	0
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	5,138
Available-for-Sale Market Value
Investment securities available-for-sale	5,157
After One Year Through Five Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	0
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	0
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	55,648
Available-for-Sale Market Value
Investment securities available-for-sale	56,434
After Five Years Through Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	4,797
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	5,476
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	148,614
Available-for-Sale Market Value
Investment securities available-for-sale	147,686
After Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	5,999
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	6,150
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	594,125
Available-for-Sale Market Value
Investment securities available-for-sale	620,883
Residential Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	19,832	20,818
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	20,878	20,766
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	419,423
Available-for-Sale Market Value
Investment securities available-for-sale	438,419
Commercial Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	97,073	101,267
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	98,369	100,613
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	506,343
Available-for-Sale Market Value
Investment securities available-for-sale	504,295
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	9,043	9,763
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	9,192	9,655
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	745,649
Available-for-Sale Market Value
Investment securities available-for-sale	772,015
Asset-backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	0	0
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	0	$ 0
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	382,415
Available-for-Sale Market Value
Investment securities available-for-sale	$ 363,799