INVESTMENT SECURITIES - Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2022	Dec. 31, 2021
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 6,191,809	$ 4,980,102
Gross unrealized gains	9,766	65,387
Gross unrealized losses	414,417	111,279
Fair value	5,787,158	4,934,210
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	6,191,809
Debt Securities, Available-for-sale	5,787,158
U.S. Treasury and Agency Securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	5,200,931	4,064,624
Gross unrealized gains	4,301	39,997
Gross unrealized losses	373,145	106,057
Debt Securities, Available-for-sale	4,832,087	3,998,564
Obligations of State and Political Subdivisions
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	488,659	413,019
Gross unrealized gains	4,220	17,326
Gross unrealized losses	16,724	56
Debt Securities, Available-for-sale	476,155	430,289
Government Agency Mortgage-Backed Securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	476,790	474,506
Gross unrealized gains	1,213	7,615
Gross unrealized losses	23,756	5,065
Debt Securities, Available-for-sale	454,247	477,056
Corporate debt securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	8,000	8,000
Gross unrealized gains	0	39
Gross unrealized losses	503	77
Debt Securities, Available-for-sale	7,497	7,962
Government Agency Collateralized Mortgage Obligations
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	17,429	19,953
Gross unrealized gains	32	410
Gross unrealized losses	289	24
Debt Securities, Available-for-sale	17,172	20,339
Federal Reserve Bank Stock [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Gross unrealized losses	0	0
Other Equity Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Gross unrealized losses	0	0
Federal Home Loan Bank Common Stock [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
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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