INVESTMENT SECURITIES - Composition, Repricing and Yield Information of Investment Securities Portfolio (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, one year or less, fair value	$ 820,913
Securities available-for-sale, one year or less, weighted average yield (percent)	1.74%
Securities available-for-sale, more than one year through five years, fair value	$ 141,459
Securities available-for-sale, more than one year through five years, weighted average yield (percent)	4.77%
Securities available-for-sale, more than five years through ten years, fair value	$ 101,790
Securities available-for-sale, more than five years through ten years, weighted average yield (percent)	2.41%
Securities available-for-sale, more than ten years, fair value	$ 181,705
Securities available-for-sale, more than ten years, weighted average yield (percent)	1.93%
Securities available-for-sale, fair value	$ 1,245,867
Securities available-for-sale, weighted average yield (percent)	2.16%
SBA loan pool securities
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, one year or less, fair value	$ 17,807
Securities available-for-sale, one year or less, weighted average yield (percent)	1.79%
Securities available-for-sale, more than one year through five years, fair value	$ 0
Securities available-for-sale, more than one year through five years, weighted average yield (percent)	0.00%
Securities available-for-sale, more than five years through ten years, fair value	$ 0
Securities available-for-sale, more than five years through ten years, weighted average yield (percent)	0.00%
Securities available-for-sale, more than ten years, fair value	$ 0
Securities available-for-sale, more than ten years, weighted average yield (percent)	0.00%
Securities available-for-sale, fair value	$ 17,807
Securities available-for-sale, weighted average yield (percent)	1.79%
U.S. government agency and U.S. government sponsored enterprise residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, one year or less, fair value	$ 0
Securities available-for-sale, one year or less, weighted average yield (percent)	0.00%
Securities available-for-sale, more than one year through five years, fair value	$ 0
Securities available-for-sale, more than one year through five years, weighted average yield (percent)	0.00%
Securities available-for-sale, more than five years through ten years, fair value	$ 30,575
Securities available-for-sale, more than five years through ten years, weighted average yield (percent)	2.20%
Securities available-for-sale, more than ten years, fair value	$ 76,856
Securities available-for-sale, more than ten years, weighted average yield (percent)	2.35%
Securities available-for-sale, fair value	$ 107,431
Securities available-for-sale, weighted average yield (percent)	2.31%
U.S. government agency and U.S. government sponsored enterprise collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, one year or less, fair value	$ 117,175
Securities available-for-sale, one year or less, weighted average yield (percent)	0.71%
Securities available-for-sale, more than one year through five years, fair value	$ 11,499
Securities available-for-sale, more than one year through five years, weighted average yield (percent)	2.01%
Securities available-for-sale, more than five years through ten years, fair value	$ 44,743
Securities available-for-sale, more than five years through ten years, weighted average yield (percent)	1.38%
Securities available-for-sale, more than ten years, fair value	$ 44,979
Securities available-for-sale, more than ten years, weighted average yield (percent)	0.39%
Securities available-for-sale, fair value	$ 218,396
Securities available-for-sale, weighted average yield (percent)	0.85%
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, one year or less, fair value	$ 0
Securities available-for-sale, one year or less, weighted average yield (percent)	0.00%
Securities available-for-sale, more than one year through five years, fair value	$ 0
Securities available-for-sale, more than one year through five years, weighted average yield (percent)	0.00%
Securities available-for-sale, more than five years through ten years, fair value	$ 9,526
Securities available-for-sale, more than five years through ten years, weighted average yield (percent)	2.60%
Securities available-for-sale, more than ten years, fair value	$ 59,707
Securities available-for-sale, more than ten years, weighted average yield (percent)	2.62%
Securities available-for-sale, fair value	$ 69,233
Securities available-for-sale, weighted average yield (percent)	2.62%
Non-agency residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, one year or less, fair value	$ 0
Securities available-for-sale, one year or less, weighted average yield (percent)	0.00%
Securities available-for-sale, more than one year through five years, fair value	$ 0
Securities available-for-sale, more than one year through five years, weighted average yield (percent)	0.00%
Securities available-for-sale, more than five years through ten years, fair value	$ 0
Securities available-for-sale, more than five years through ten years, weighted average yield (percent)	0.00%
Securities available-for-sale, more than ten years, fair value	$ 163
Securities available-for-sale, more than ten years, weighted average yield (percent)	6.35%
Securities available-for-sale, fair value	$ 163
Securities available-for-sale, weighted average yield (percent)	6.35%
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, one year or less, fair value	$ 685,931
Securities available-for-sale, one year or less, weighted average yield (percent)	1.91%
Securities available-for-sale, more than one year through five years, fair value	$ 0
Securities available-for-sale, more than one year through five years, weighted average yield (percent)	0.00%
Securities available-for-sale, more than five years through ten years, fair value	$ 0
Securities available-for-sale, more than five years through ten years, weighted average yield (percent)	0.00%
Securities available-for-sale, more than ten years, fair value	$ 0
Securities available-for-sale, more than ten years, weighted average yield (percent)	0.00%
Securities available-for-sale, fair value	$ 685,931
Securities available-for-sale, weighted average yield (percent)	1.91%
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, one year or less, fair value	$ 0
Securities available-for-sale, one year or less, weighted average yield (percent)	0.00%
Securities available-for-sale, more than one year through five years, fair value	$ 129,960
Securities available-for-sale, more than one year through five years, weighted average yield (percent)	5.01%
Securities available-for-sale, more than five years through ten years, fair value	$ 16,946
Securities available-for-sale, more than five years through ten years, weighted average yield (percent)	5.73%
Securities available-for-sale, more than ten years, fair value	$ 0
Securities available-for-sale, more than ten years, weighted average yield (percent)	0.00%
Securities available-for-sale, fair value	$ 146,906
Securities available-for-sale, weighted average yield (percent)	5.08%