INVESTMENTS - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	$ 3,000,986,000	$ 2,450,265,000
Gross Unrealized Gain	50,603,000	80,871,000
Gross Unrealized Loss	63,704,000	2,079,000
Allowance for Credit Losses	0	0
Investment securities available-for-sale	2,987,885,000	2,529,057,000
Held-to-Maturity Debt Securities
Investment securities held-to-maturity, net	103,528,000	111,747,000
Gross Unrealized Gain	4,107,000	4,680,000
Gross Unrealized Loss	1,000	0
Allowance for credit loss	5,000	6,000
Fair Value	107,629,000	116,421,000
State and political subdivisions
Held-to-Maturity Debt Securities
Investment securities held-to-maturity, net	103,027,000	111,246,000
Gross Unrealized Gain	4,107,000	4,678,000
Gross Unrealized Loss	0	0
Allowance for credit loss	5,000	6,000	$ 8,000	$ 0
Fair Value	107,129,000	115,918,000
Available for sale securities transfers to held to maturity unrealized gains	300,000	400,000
Foreign bonds
Held-to-Maturity Debt Securities
Investment securities held-to-maturity, net	501,000	501,000
Gross Unrealized Gain	0	2,000
Gross Unrealized Loss	1,000	0
Allowance for credit loss	0	0
Fair Value	500,000	503,000
CMO
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	564,055,000	461,819,000
Gross Unrealized Gain	5,604,000	9,949,000
Gross Unrealized Loss	15,223,000	443,000
Allowance for Credit Losses	0	0
Investment securities available-for-sale	554,436,000	471,325,000
FNMA MBS
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	1,997,140,000	1,544,105,000
Gross Unrealized Gain	35,151,000	55,747,000
Gross Unrealized Loss	34,246,000	882,000
Allowance for Credit Losses	0	0
Investment securities available-for-sale	1,998,045,000	1,598,970,000
FHLMC MBS
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	182,271,000	190,856,000
Gross Unrealized Gain	8,941,000	12,142,000
Gross Unrealized Loss	759,000	105,000
Allowance for Credit Losses	0	0
Investment securities available-for-sale	190,453,000	202,893,000
GNMA MBS
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	25,011,000	22,716,000
Gross Unrealized Gain	907,000	1,046,000
Gross Unrealized Loss	83,000	0
Allowance for Credit Losses	0	0
Investment securities available-for-sale	25,835,000	23,762,000
GSE agency notes
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	232,509,000	230,769,000
Gross Unrealized Gain	0	1,987,000
Gross Unrealized Loss	13,393,000	649,000
Allowance for Credit Losses	0	0
Investment securities available-for-sale	$ 219,116,000	$ 232,107,000