INVESTMENT SECURITIES, Investment Securities in a Continuous Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	218	117
12 months or longer	3	3
Total	221	120
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 1,634,895	$ 913,612
12 months or longer	13	14
Total	1,634,908	913,626
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	179,439	16,788
12 months or longer	0	0
Total	179,439	$ 16,788
Available-for-sale, allowance for credit losses	0
Accrued interest and fees receivable	20,900
Available-for-sale Securities [Member]
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Accrued interest and fees receivable	$ 12,000
U.S. Treasury and agency securities
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	31	13
12 months or longer	0	0
Total	31	13
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 1,440,822	$ 831,015
12 months or longer	0	0
Total	1,440,822	831,015
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	174,454	16,595
12 months or longer	0	0
Total	$ 174,454	$ 16,595
Obligations of state and political subdivisions
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	17	1
12 months or longer	0	0
Total	17	1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 7,028	$ 358
12 months or longer	0	0
Total	7,028	358
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	9	0
12 months or longer	0	0
Total	$ 9	$ 0
Government agency mortgage-backed securities
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	162	89
12 months or longer	2	2
Total	164	91
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 184,325	$ 75,992
12 months or longer	13	14
Total	184,338	76,006
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	4,969	182
12 months or longer	0	0
Total	$ 4,969	$ 182
Corporate debt securities
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months		1
12 months or longer		0
Total		1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months		$ 1,001
12 months or longer		0
Total		1,001
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months		1
12 months or longer		0
Total		$ 1
Government agency collateralized mortgage obligations
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months	8	13
12 months or longer	1	1
Total	9	14
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 2,720	$ 5,246
12 months or longer	0	0
Total	2,720	5,246
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	7	10
12 months or longer	0	0
Total	$ 7	$ 10