INVESTMENT SECURITIES - Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2023 USD ($) security	Dec. 31, 2022 USD ($) security
Fair Value
Less than 12 Months	$ 99,430	$ 1,423,379
12 Months or Longer	1,400,671	487,129
Total	1,500,101	1,910,508
Gross Unrealized Losses
Less than 12 Months	10,826	172,828
12 Months or Longer	371,972	124,297
Total	$ 382,798	$ 297,125
Number of Securities | security	936	1,154
U.S. Treasury
Fair Value
Less than 12 Months	$ 397	$ 2,459
12 Months or Longer	1,316	0
Total	1,713	2,459
Gross Unrealized Losses
Less than 12 Months	1	42
12 Months or Longer	23	0
Total	$ 24	$ 42
Number of Securities | security	3	5
U.S. Government-sponsored agency securities
Fair Value
Less than 12 Months	$ 796	$ 48,940
12 Months or Longer	92,789	53,022
Total	93,585	101,962
Gross Unrealized Losses
Less than 12 Months	3	4,973
12 Months or Longer	20,826	12,219
Total	$ 20,829	$ 17,192
Number of Securities | security	16	16
State and municipal
Fair Value
Less than 12 Months	$ 86,964	$ 1,177,104
12 Months or Longer	886,092	108,652
Total	973,056	1,285,756
Gross Unrealized Losses
Less than 12 Months	10,369	150,096
12 Months or Longer	233,868	28,630
Total	$ 244,237	$ 178,726
Number of Securities | security	743	946
U.S. Government-sponsored mortgage-backed securities
Fair Value
Less than 12 Months	$ 11,273	$ 182,700
12 Months or Longer	408,686	325,455
Total	419,959	508,155
Gross Unrealized Losses
Less than 12 Months	453	16,910
12 Months or Longer	116,114	83,448
Total	$ 116,567	$ 100,358
Number of Securities | security	164	177
Corporate obligations
Fair Value
Less than 12 Months	$ 0	$ 12,176
12 Months or Longer	11,788	0
Total	11,788	12,176
Gross Unrealized Losses
Less than 12 Months	0	807
12 Months or Longer	1,141	0
Total	$ 1,141	$ 807
Number of Securities | security	10	10