INVESTMENT SECURITIES - Investment Securities in a Continuous Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	182	117
12 months or longer	29	3
Total	211	120
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 1,415,835	$ 913,612
12 months or longer	597,935	14
Total	2,013,770	913,626
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	44,998	16,788
12 months or longer	77,405	0
Total	122,403	$ 16,788
Available-for-sale, allowance for credit losses	0
Accrued interest and fees receivable	18,200
Available-for-sale Securities [Member]
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Accrued interest and fees receivable	$ 12,500
U.S. Treasury and Agency Securities [Member]
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	38	13
12 months or longer	8	0
Total	46	13
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 1,223,230	$ 831,015
12 months or longer	579,793	0
Total	1,803,023	831,015
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	41,243	16,595
12 months or longer	76,829	0
Total	$ 118,072	$ 16,595
Obligations of State and Political Subdivisions [Member]
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	21	1
12 months or longer	0	0
Total	21	1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 16,608	$ 358
12 months or longer	0	0
Total	16,608	358
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	220	0
12 months or longer	0	0
Total	$ 220	$ 0
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	117	89
12 months or longer	20	2
Total	137	91
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 169,915	$ 75,992
12 months or longer	18,069	14
Total	187,984	76,006
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	3,528	182
12 months or longer	576	0
Total	$ 4,104	$ 182
Corporate debt securities
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	1	1
12 months or longer	0	0
Total	1	1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 4,994	$ 1,001
12 months or longer	0	0
Total	4,994	1,001
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	6	1
12 months or longer	0	0
Total	$ 6	$ 1
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	5	13
12 months or longer	1	1
Total	6	14
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 1,088	$ 5,246
12 months or longer	73	0
Total	1,161	5,246
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	1	10
12 months or longer	0	0
Total	$ 1	$ 10