INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 156,262
Due after one through five years	876,849
Due after five years through ten years	1,958,202
Due after ten years	1,494,330
Subtotal	4,485,643
Amortized cost	4,980,102
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	158,119
Due after one through five years	891,772
Due after five years through ten years	1,962,453
Due after ten years	1,424,471
Subtotal	4,436,815
Fair value	4,934,210
Government Agency Mortgage-Backed Securities
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	474,506	$ 506,540
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	477,056	522,638
Government Agency Collateralized Mortgage Obligations
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	19,953	42,476
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	20,339	43,577
U.S. Treasury and Agency Securities
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	4,064,624	2,423,236
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	$ 3,998,564	$ 2,501,382

Amortized cost of 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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