INVESTMENT SECURITIES - Available-for-sale portfolio (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021
Available-for-Sale Portfolio
Amortized cost	$ 4,675,474	$ 4,980,102
Gross unrealized gains	565	65,387
Gross unrealized losses	524,188	111,279
Total - Fair value	4,151,851	4,934,210
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	4,675,474	4,980,102
Total - Fair value	4,151,851	4,934,210
U.S. Treasury and agency securities
Available-for-Sale Portfolio
Amortized cost	3,660,546	4,064,624
Total - Fair value	3,243,537	3,998,564
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	3,660,546	4,064,624
Gross unrealized gains	0	39,997
Gross unrealized losses	417,009	106,057
Total - Fair value	3,243,537	3,998,564
Obligations of state and political subdivisions
Available-for-Sale Portfolio
Amortized cost	549,118	413,019
Total - Fair value	504,297	430,289
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	549,118	413,019
Gross unrealized gains	506	17,326
Gross unrealized losses	45,327	56
Total - Fair value	504,297	430,289
Government agency mortgage-backed securities
Available-for-Sale Portfolio
Amortized cost	444,689	474,506
Total - Fair value	384,633	477,056
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	444,689	474,506
Gross unrealized gains	58	7,615
Gross unrealized losses	60,114	5,065
Total - Fair value	384,633	477,056
Corporate debt securities
Available-for-Sale Portfolio
Amortized cost	8,000	8,000
Total - Fair value	7,114	7,962
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	8,000	8,000
Gross unrealized gains	0	39
Gross unrealized losses	886	77
Total - Fair value	7,114	7,962
Government agency collateralized mortgage obligations
Available-for-Sale Portfolio
Amortized cost	13,121	19,953
Total - Fair value	12,270	20,339
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	13,121	19,953
Gross unrealized gains	1	410
Gross unrealized losses	852	24
Total - Fair value	12,270	20,339
Federal reserve bank common stock
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Gross unrealized losses	0	0
Other equity securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Gross unrealized losses	0	0
Federal home loan bank common stock
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Gross unrealized losses	$ 0	$ 0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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