INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Held-to-Maturity Amortized Cost
Amortized Cost	$ 92,597	$ 98,420
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	90,001	99,898
Available-for-Sale Amortized Cost
Debt Securities, Available-for-sale, Amortized Cost	4,113,169	4,180,589
Available-for-Sale Market Value
Investment securities available-for-sale	3,957,882	4,207,846
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
Debt Securities, Available-for-sale, Amortized Cost	11,539
Available-for-Sale Market Value
Investment securities available-for-sale	11,556
After One Year Through Five Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	1,072
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	1,092
Available-for-Sale Amortized Cost
Debt Securities, Available-for-sale, Amortized Cost	89,910
Available-for-Sale Market Value
Investment securities available-for-sale	89,562
After Five Years Through Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	36,271
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	35,340
Available-for-Sale Amortized Cost
Debt Securities, Available-for-sale, Amortized Cost	357,140
Available-for-Sale Market Value
Investment securities available-for-sale	348,300
After Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	2,263
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	2,268
Available-for-Sale Amortized Cost
Debt Securities, Available-for-sale, Amortized Cost	854,887
Available-for-Sale Market Value
Investment securities available-for-sale	816,480
Residential Mortgage 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
Debt Securities, Available-for-sale, Amortized Cost	709,888	728,050
Available-for-Sale Market Value
Investment securities available-for-sale	667,072	724,137
Commercial Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	42,211	46,362
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	40,808	47,013
Available-for-Sale Amortized Cost
Debt Securities, Available-for-sale, Amortized Cost	754,978	729,948
Available-for-Sale Market Value
Investment securities available-for-sale	737,430	731,890
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	10,780	11,882
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	10,493	12,103
Available-for-Sale Amortized Cost
Debt Securities, Available-for-sale, Amortized Cost	621,240	696,258
Available-for-Sale Market Value
Investment securities available-for-sale	594,855	697,740
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
Debt Securities, Available-for-sale, Amortized Cost	713,587	720,638
Available-for-Sale Market Value
Investment securities available-for-sale	$ 692,627	$ 719,581