INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 496,539
Due after one through five years	1,497,928
Due after five years through ten years	2,119,643
Due after ten years	1,659,336
Subtotal	5,773,446
Amortized cost	6,255,259	$ 4,980,102
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	495,239
Due after one through five years	1,424,444
Due after five years through ten years	1,922,167
Due after ten years	1,308,082
Subtotal	5,149,932
Fair value	5,590,427	4,934,210
Government Agency Mortgage-Backed Securities
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	466,276
Amortized cost	466,276	474,506
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	425,357
Fair value	425,357	477,056
Government Agency Collateralized Mortgage Obligations
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	15,537
Amortized cost	15,537	19,953
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	15,138
Fair value	$ 15,138	$ 20,339