INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Held-to-Maturity Amortized Cost
Amortized Cost	$ 127,347	$ 142,862
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	132,956	142,821
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	2,815,199	2,798,298
Available-for-Sale Market Value
Investment securities available-for-sale	2,897,413
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	4,126
Available-for-Sale Market Value
Investment securities available-for-sale	4,162
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	55,428
Available-for-Sale Market Value
Investment securities available-for-sale	56,534
After Five Years Through Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	4,840
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	5,720
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	153,966
Available-for-Sale Market Value
Investment securities available-for-sale	156,133
After Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	5,819
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	6,076
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	587,481
Available-for-Sale Market Value
Investment securities available-for-sale	617,848
Residential Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	18,560	20,818
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	18,924	20,766
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	413,981
Available-for-Sale Market Value
Investment securities available-for-sale	431,523
Commercial Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	90,005	101,267
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	94,005	100,613
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	489,962
Available-for-Sale Market Value
Investment securities available-for-sale	494,557
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	8,123	9,763
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	8,231	9,655
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	729,540
Available-for-Sale Market Value
Investment securities available-for-sale	760,598
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	380,715
Available-for-Sale Market Value
Investment securities available-for-sale	$ 376,058