INVESTMENT SECURITIES - Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2023	Dec. 31, 2022
Available-for-Sale Portfolio
Amortized Cost	$ 3,314,815	$ 4,675,474
Gross Unrealized Gains	26	565
Gross Unrealized Losses	547,205	524,188
Fair Value	2,767,636	4,151,851
Held-to-Maturity Portfolio
Amortized Cost	1,125,540	1,079,695
Gross Unrealized Gains	0	0
Gross Unrealized Losses	134,880	44,900
Fair Value	990,660	1,034,795
Equity Securities, at Fair Value
Amortized Cost	251	251
Gross Unrealized Gains	69	168
Gross Unrealized Losses	0	0
Fair Value	320	419
U.S. Treasury and agency securities
Available-for-Sale Portfolio
Amortized Cost	2,378,607	3,660,546
Gross Unrealized Gains	0	0
Gross Unrealized Losses	399,168	417,009
Fair Value	1,979,439	3,243,537
Held-to-Maturity Portfolio
Amortized Cost	1,101,679	1,079,695
Gross Unrealized Gains	0	0
Gross Unrealized Losses	134,091	44,900
Fair Value	967,588	1,034,795
Obligations of state and political subdivisions
Available-for-Sale Portfolio
Amortized Cost	506,838	549,118
Gross Unrealized Gains	20	506
Gross Unrealized Losses	72,393	45,327
Fair Value	434,465	504,297
Government agency mortgage-backed securities
Available-for-Sale Portfolio
Amortized Cost	411,083	444,689
Gross Unrealized Gains	6	58
Gross Unrealized Losses	74,018	60,114
Fair Value	337,071	384,633
Held-to-Maturity Portfolio
Amortized Cost	23,861	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	789	0
Fair Value	23,072	0
Corporate debt securities
Available-for-Sale Portfolio
Amortized Cost	8,000	8,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	826	886
Fair Value	7,174	7,114
Government agency collateralized mortgage obligations
Available-for-Sale Portfolio
Amortized Cost	10,287	13,121
Gross Unrealized Gains	0	1
Gross Unrealized Losses	800	852
Fair Value	9,487	$ 12,270
Held-to-Maturity Portfolio
Fair Value	$ 0

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