INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 198,566
Due after one through five years	768,349
Due after five years through ten years	1,018,820
Due after ten years	1,504,697
Subtotal	3,490,432
Amortized cost	4,039,978
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	199,557
Due after one through five years	796,228
Due after five years through ten years	1,037,749
Due after ten years	1,420,784
Subtotal	3,454,318
Fair value	4,012,739
Government agency mortgage-backed securities
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	521,056
Amortized cost	521,056	$ 506,540
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	529,139
Fair value	529,139	522,638
Government agency collateralized mortgage obligations
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	28,490
Amortized cost	28,490	42,476
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	29,282
Fair value	$ 29,282	$ 43,577