INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 152,902
Due after one through five years	1,801,393
Due after five years through ten years	2,163,695
Due after ten years	1,579,600
Subtotal	5,697,590
Amortized cost	6,191,809
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	153,690
Due after one through five years	1,757,237
Due after five years through ten years	2,038,048
Due after ten years	1,366,764
Subtotal	5,315,739
Fair value	5,787,158
Government Agency Mortgage-Backed Securities
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	476,790
Amortized cost	476,790	$ 474,506
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	454,247
Fair value	454,247	477,056
Government Agency Collateralized Mortgage Obligations
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	17,429
Amortized cost	17,429	19,953
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	17,172
Fair value	$ 17,172	$ 20,339