INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Held-to-Maturity Amortized Cost
Amortized Cost	$ 118,072	$ 142,862
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	123,441	142,821
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	2,915,937	2,798,298
Available-for-Sale Market Value
Investment securities available-for-sale	3,004,963
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	4,098
Available-for-Sale Market Value
Investment securities available-for-sale	4,133
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	53,902
Available-for-Sale Market Value
Investment securities available-for-sale	55,486
After Five Years Through Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	5,784
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	6,864
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	183,872
Available-for-Sale Market Value
Investment securities available-for-sale	190,863
After Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	4,531
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	4,643
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	615,870
Available-for-Sale Market Value
Investment securities available-for-sale	648,572
Residential Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	16,069	20,818
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	16,319	20,766
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	393,395
Available-for-Sale Market Value
Investment securities available-for-sale	409,420
Commercial Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	84,701	101,267
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	88,535	100,613
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	550,408
Available-for-Sale Market Value
Investment securities available-for-sale	556,560
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	6,987	9,763
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	7,080	9,655
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	680,581
Available-for-Sale Market Value
Investment securities available-for-sale	706,133
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	433,811
Available-for-Sale Market Value
Investment securities available-for-sale	$ 433,796