INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 187,457
Due after one through five years	693,642
Due after five years through ten years	1,506,410
Due after ten years	1,492,075
Subtotal	3,879,584
Amortized cost	4,406,878
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	189,642
Due after one through five years	715,779
Due after five years through ten years	1,515,582
Due after ten years	1,402,300
Subtotal	3,823,303
Fair value	4,358,371
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	503,548
Amortized cost	503,548	$ 506,540
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	510,676
Fair value	510,676	522,638
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	23,746
Amortized cost	23,746	42,476
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	24,392
Fair value	$ 24,392	$ 43,577